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Consumers of BCD probably shouldn't use the experimental field.

BCD tables on MDN show an icon for experimental features. We could instead show a Baseline icon, but we would probably have to compute it directly from each BCD feature, without taking into consideration web-features.

I would propose to add this computed Baseline status per BCD feature into the published BCD data, and to do this at the same time of deprecating experimental.

I don't think we should wait. This advice would be to new consumers of BCD: they should not use experimental for any purpose. But existing consumers of BCD may (at least until there's a plan to remove experimental).

We had a lengthy discussion about this on today's BCD call, which prompted me to revise this PR. Some highlights from that discussion:

• I explained a bit more about why I want to see this deprecation notice appear. My motivation here is to discourage new uses for the experimental data by new consumers of BCD. I do not intend to disrupt MDN or other existing consumers' use of experimental (unless and until they switch to alternatives).
• I think @pepelsbey suggested that I confirm with Ruth that MDN content is fine with this, the next time I see her. Since there is no plan to remove this as yet, I don't expect any problems with declaring this deprecated.
• We had a lengthy conversation—one might say "debate"—about the meaning of the experimental field. I've revised the PR to make it more clear that the experimental field has a murky history from MDN's days as a wiki and an evolving meaning. This prompted the revisions to the PR.
• @Elchi3 pointed out that declaring the deprecation would also get us new information, feedback from consumers, by announcing the deprecation.

I've revised the PR to make it more clear in the documentation that experimental's meaning has been loose over its history.

I'll also add here that if we merge this PR, the following release should be semver minor and there should be a notice in the release notes about the change wrought by this PR.

From an MDN perspective marking it deprecated won't affect us, as long as we can keep using it (and adding it to bcd) until we can replace the labelling on our pages.

We do want to do this, however I think it will be a bit of a timely journey - I need to to do a full review but currently a rough timeline looks something like

• Deal with 'deprecated' first, alongside the addition of 'not recommended' baseline (this gives us a good understanding of what is entailed in removing current labelling)
• Prototype icon replacement (sidebars, compat tables) as these are also affected
• Assess and make a plan for experimental