As I understand it, starting to use TS4 only features would break all TS3 users, correct?
If so: this sounds like the original issue we had when adding types. As our new types used TS3, folks on earlier TS versions who (even when deep down) depended on our projects, couldn’t compile anymore?

As I understand it, starting to use TS4 only features would break all TS3 users, correct?

It would break TS 3 users directly depending on unified's typings or who have strict lib checks enabled

If so: this sounds like the original issue we had when adding types. As our new types used TS3, folks on earlier TS versions who (even when deep down) depended on our projects, couldn’t compile anymore?

The frustration previously was from introducing a typescript version requirement/upgrade in a minor release.
This would introduce the change in a major release.
Which means version spreads ~ (patch), and ^ (minor) would not pick up these changes.

Combined a clear changelog, this allows downstream adopters to choose when they want to upgrade.
For most users TypeScript version upgrades are seamless, and using this update would be seemless.
Any errors thrown would be legitimate cases where the settings object don't match up with the plugin.

Another option on available.
It is possible to support multiple versions of the typings using https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html#version-selection-with-typesversions
We could publish a v3.2 and v4.0 version initially, and remove the v3.2 in a major release.
My concern would be the maintenance cost of having more than one version of the typings.

To me maintaining two versions for a bit doesn’t seem too bad. It allows us to push TS 4 earlier and to more folks (it doesn’t need to bubble through remark/rehype/retext). While the load of maintaining two versions shouldn’t be very time consuming (some copy/pasting, and we don’t touch them too much after creation). Then, say in a year or so, when a next major is due to some other reason, we can ditch TS 3.2?

To me maintaining two versions for a bit doesn’t seem too bad. It allows us to push TS 4 earlier and to more folks (it doesn’t need to bubble through remark/rehype/retext). While the load of maintaining two versions shouldn’t be very time consuming (some copy/pasting, and we don’t touch them too much after creation).

Done, the types have been split.

Good to go? Minor?

Yep, good to go, and semver minor should be accurate.

Released in 9.2.0!

Not sure if it's related to this change or not, but just so you know, I got this error after upgrading to v9.2.0:

node_modules/rehype-react/types/index.d.ts:3:27 - error TS7016: Could not find a declaration file for module 'unified'. '/Users/haz/Projects/reakit/node_modules/unified/index.js' implicitly has an 'any' type.
  Try `npm install @types/unified` if it exists or add a new declaration (.d.ts) file containing `declare module 'unified';`

3 import {Transformer} from 'unified'
                            ~~~~~~~~~


Found 1 error.

I'm not using TS 4 yet. Solution was to use 9.1.0.

@diegohaz could you open a thread in https://github.com/unifiedjs/unified/discussions?
and share your tsconfig + package.json, or even better a runnable example of the issue in https://codesandbox.io ?

This change has been tested across typescript versions from 3.4 to 4.0 using dtslint, both in this repo, and again downstream in most remark/rehype plugins that provide typings (for example remark itself)
Quite a few cases are covered by the test suite, if this does turn out to be an issue, the details around tsconfig and surrounding tooling will be needed to track it down.