I'm going to start out by saying that what's below is just some stuff I feel like needs to be said and not all of it is directed at SO. I've probably said some of it before but I see these little nuggets all over and I'd like to push back some because I think they're getting in the way of any viable discussion between the company and the community.
The Question
While closure was an effective tool to manage higher volume in the past, we believe it is now a significant cause of friction to authors and curators.
The data doesn't support that closure/review has ever been an effective tool on high-volume sites and, I'd argue, is actually better-suited to medium sized sites. The close review queue is (was, anyway) tuned to have an upper number of questions in the queue and (if I'm remembering correctly) actually has a variable in the "aging" formula to kick questions out of review sooner if the queue gets too big. Five digit numbers are demotivating for reviewers, y'know?
Historically there have been significantly more posts nominated for closure but aging out of review than getting closed. This led to the number of votes needed to close being reduced from 5 to 3 back in 2019, as only 36% of questions that received a first vote to close would complete the process when 5 votes to close were required. Even after the change, nearly half of questions nominated for closure aged out of review.
Unfortunately, for small sites, the review queue indicators don't always light up to draw attention to close reviews, leading to those sites also struggling with closure. Review works best on sites with enough questions getting reviewed for indicators to work but not so many that the community of reviewers can't handle them before they age out.
As such, I'd say SO is probably nearly at the sweet spot for review to actually work again, ironically. Though that assumes there are reviewers to handle those questions.
We want to differentiate “this content doesn’t belong on Stack Overflow” from “this isn’t a question I am interested in,” which is different from “this is high-quality content that is worth promoting”. We want tooling that lets those signals exist independently, and we want curator input on what that actually looks like in practice.
I don't actually have a big problem with this, though you're outlining three axes with statements that relate to one end of each axis instead of looking at the entire axes as a whole.
More accurately, the three axes are:
- Scope - ranges from out of scope to in scope. This can be indicated through comments, votes and - in the case of out of scope content - closure/deletion.
- Interest - ranges from disinterest to interest. Interest can be indicated through "saves" and following, though disinterest can only be indicated on a tag-wide basis, through ignoring tags. I'd say it's not uncommon for users to indicate strong disinterest with downvotes.
- Quality - ranges from low quality to high quality. The primary indicators here are comments and votes, though flags/closure/deletion are options in some cases.
Scope is really broad and often subjective. Why doesn't the content belong? Is it about edible cookies? Is it a post type that isn't in-scope here? Is it in a non-English language? Do community members even agree about whether the question belongs here? This is generally determined by the community and the goal should be to be as inclusive as is reasonable. That said, the community is empowered to determine questions - or often question types - that don't perform well in the Q&A format.
Interest is... interesting because the platform doesn't generally put much value in whether or not someone finds a specific question interesting. The (unstated) expectation is that votes and closure shouldn't be used to indicate that a question is or isn't "interesting" - many basic questions may be "uninteresting" or even "useless" to an expert but that doesn't mean they should be downvoted - but I'd say they are used for this in many cases, couched as "too basic" or "doesn't show research effort". That said, I'd argue that in some cases, duplicate closure is overused for "basic" or "uninteresting" questions far more often than is warranted.
Quality is where votes should come in but this is also subjective, particularly without adequate guidance for when to vote. For example, the hover text for voting heavily relies on "useful" without qualifying who should find it useful, which leads to people calling things they personally find uninteresting "not useful" and then downvoting it. The other element of voting on questions is "shows research effort", which is quite subjective and also used problematically at times. I'm not quite sure how we ended up with these two metrics being how we define "quality" but, here we are.
Here's the thing. You are welcome to propose as many different methods for giving feedback as you like but you can't actually force people to use them "correctly". People will use whatever tools they have at hand to make content they don't like go away, even if that's not the purpose of the tool. Another answer mentions the fact that sufficiently downvoted posts are removed from the front page - has that feature incentivised people to downvote more to hide content they don't want? Probably.
Please - try to make this better, more clear, more "useful" but don't expect people to follow those intentions, particularly if you continue to do nothing to reduce the volume of content that needs curation.
Consider the following:
A user has an excellent answer explaining concept A. It's highly regarded and has been around for several years. Over the years, they see the same question asked over and over by different users who didn't find the excellent answer that already exists. They dutifully duplicate close and leave comments clarifying how their existing answer solves the problem. Over time, they get frustrated that users frequently fail to find their answer, which they've kept updated as the technology has changed.
With the question vote hover text in mind, they start downvoting these questions in addition to duplicate voting because if the asker had put in any "research effort", they would have found the existing answer and never asked the question in the first place and it's not "useful" since there's a duplicate question. Even well-asked, fully-explained questions get the downvote simply because they failed to find the existing answer. The user's once-helpful comments take on a tinge of frustration, caused by years of pointing people to a resource that already exists. Maybe the comments even stop entirely, with the questions just being duplicate closed and downvoted.
The thing is, a well-asked duplicate should be closed and it should get a helpful comment but it shouldn't get a downvote for not showing "research effort" or not being "useful"... but what do you think led to those votes? Was it the asker? Or was it the platform failing to prevent the question being asked - not just once - but repeatedly over many years?
Now, you're in a position of having these curators who are already misusing votes because the system is overburdening them and you're discussing changing the curation system that is all they have - but you literally frame it as
"we plan to retire certain curation workflows, such as close votes and most review queues, in the new design."
... and then, when the community goes on high alert you come here and, instead of asking how the curation system could better serve the community, you put out this "thought exercise" that - as many have pointed out - seems to indicate an incomplete comprehension of how curation on the platform functions.
We are early in the development life cycle with these changes, and we're actively looking for experienced community members to pressure-test these ideas in a workshop type of way with the product manager and designer that has been working on close reasons.
You're going at it backwards. Stop asking the community to review your solutions and give us a chance to actually get your PM/designer up to speed without having to tear down a preconceived idea of how to "solve" this issue. This is a huge ecosystem and solving curation issues without solving (or at least fully understanding) the content creation issues that necessitate the curation tooling in the first place will lead to failure and frustration.
Remember, closure isn't the end of things, either - it leads to deletion. Without question closure, there will be a ton more content on the site than there currently is. If you look at closed questions over a year old with no answers (non-duplicate), there are only 9.6k, most of which have a score of 2 or higher. Compare that to these 1.6 million zero score open questions with no answers over a year old that have been saved from deletion by simply having two or more comments - that's over half of all unanswered open questions! The platform would be full of significantly more cruft without closure or a path to deletion for content that isn't engaging to people.
Comments/Answers
Votes vs closure
Why do you conflate downvotes and question closure? These are two different mechanisms with totally different purpose! ~ Dharman
It's completely reasonable for the company to consider these two together because the user experience for the asker whose question is closed almost always includes downvotes. Heck, Anerdw even draws the equivalence:
"Structured downvote reasons" already exist. They're called close reasons.
There are 277 non-deleted questions that were asked in the last 10 days that have been closed for non-duplicate reasons. Of them, 205 have a negative score, 161 have a score of -2 or less, and 31 have a score of -5 or less. Of the 51 duplicate closures of questions asked in that time, only 16 have a negative score but all 16 have a score of -2 or less.
These two mechanisms have significantly intertwined usages since being close-worthy generally leads to downvotes and - even if closure can be overcome - the downvotes often lead to it feeling like it's not worth trying to fix the problems. Remember, while they may be signals to two different people, one person gets the full impact of both signals.
This is one of the reasons I've batted around the idea of whether downvotes on questions make sense at all - beyond their functional purpose of leading to post deletion. It's not an easy problem to solve but let's not pretend the two are unrelated.
Duplicates
When we close a question as a duplicate, it's because we are 100% sure that the answers in the linked question are the best solution. ~ Dharman
You don't even agree with this statement since your next sentence acknowledges that duplicates have been used too broadly in the past and others on this page have pointed out that duplicate closures can be wrong or point at overly-broad canonicals.
My biggest problem is probably with omnibus canonicals being used as a duplicate closure. Which of the 50+ answers will be useful to the asker? Point it out to the asker. If the question is too unclear to point it out, vote to close as unclear. It's ing stupid to tell someone with an unclear question that "Your answer is in here. Somewhere. Go dig." ~ user4581301
This is how I've felt for a long time. Canonicals can be extremely helpful but using them as a duplicate target is not a good UX for anyone. Tim Post once talked about an "answer as duplicate" option where someone could cite excerpts of an existing answer on another question while closing to ensure the germane parts were visible. This sounds similar to what's being mentioned as a solution in the question.
While user4581301 and I share similar feelings, others say things like:
The problem with duplicate closure is that users just want an exact, tailored answer. They're lazy. They don't want to learn. They want to be served. ~ Cerbrus
This frustration is understandable but it's very uncharitable to assume that these askers are "lazy", particularly after having just stated:
Duplicate targets often ask and answer the same concept, but can not be taken out of context.
Not everyone on SO is enough of an expert to take a concept and apply it to their specific situation - an "answer" that they can't use, isn't an answer. If we find that existing answers aren't helpful to users, we need to investigate whether the answers adequately generalize the solution to help users who have their question closed as a duplicate recognize it as such.
We need to stop dupe closing and pointing to questions with 50+ answers and then calling askers "lazy" for not being able to sift through thousands of characters to find the bit that actually solves their problem.
The entire point of duplicate closure is that the linked question solves the asker’s problem. If it doesn’t, then whoever closed it was just wrong and it should be reopened. No system changes are necessary. ~ Starship
It is technically true that any closed question can be reopened but it almost never actually happens. I've personally had a question closed as a duplicate here on SO and I'm pretty sure the only reason it was reopened was because I went to the Blue Room and whinged at the SO mods about my poor question being closed. Most users don't have that access.
If you have 10k, go to the Question Closed Stats and just look at the numbers:

I also, at least in my experience, find that duplicates are one of the easiest to get reopened. [...] they can ping the closer in the comments and they can reopen it if they evidenced appropriately. ~ Thom A
Closed questions almost never get reopened and, over the last 90 days anyway, duplicates get reopened far less often than other closure reasons despite being the second most common reason for closure. Considering it's the only closure type that can be unilaterally undone by a gold tag badge user, the percentage being so low seems to indicate users find little success in getting duplicate closed questions reopened.
Remember, askers - who are often newer users and low-rep - can't see who closed their posts without looking at the timeline and they don't have a way to ping the close voter unless the post was unilaterally closed... but how would they even know that? I forgot that you can ping unilateral close voters and I wrote this five years ago, so I clearly knew it in the past.
We need to stop insisting that the status quo is sufficient and transparent.
Uninteresting content
"this isn’t a question I am interested in" - Closure was never about not being interested in some particular questions. Dalija Prasnikar
"this isn’t a question I am interested in" If a user is not interested in a question they won't open it, or simply close the page after reading it. They may also downvote it after reading, but they definitely should not vote to close. Using that as a reason to close a question is an abuse of their close voting privileges and could result in a warning from the mods. Dharman
"This isn't a question I am interested in": click away, add any uninteresting tags to your ignore list if they aren't there already, possibly edit the question to add those tags if they aren't already present. kaya3
However much we might say people shouldn't downvote or vote to close questions they don't find interesting, as a community, let's not claim it doesn't happen or that people aren't using "lacks research effort" and "not useful" as code for "uninteresting" in the same way some use duplicate closure too broadly to point at omnibus answers that would require hours to digest.
I'm not saying that everyone does this but that doesn't mean it doesn't happen. I can't possibly understand how else you explain the misuse of duplicates that has persisted here and the attitudes of some when it's suggested that a little more effort be put in to ensure askers can actually find the relevant content in the duplicate.
The reality is, when you've been answering questions for years, most questions won't be novel and you're going to be as exhausted as the user described in the "consider" section above. Despite that, we need to remember that it's the platform at fault when users fail to find answers that already exist or ask questions that need improvement or otherwise qualify for closure. We need to be more willing to throw the blame at the platform and give the askers grace and remember that our expertise includes knowing how the platform works, which most people using the site don't have.
The company needs to improve the platform so that all users have a better experience but they seem unable to see themselves as the source of the problems and seem to require community members to pick up more of the effort as they revamp the system without actually making - or even understanding - the curation process.
Stop asserting that the platform works fine as it is - it doesn't! Question whether the reality aligns with the intention rather than assuming it does or being blind when it does not. We don't have to roll over and accept the solutions the company is providing but when we can't recognize the flaws in the system and accept that things need to change, the company has no reason to try and partner with us to solve the problems.