[META] Stabilise and/or remove experimental modules as appropriate in/before Drupal 9

So currently we add modules to core at alpha, and remove them from branches until they reach beta stability, after which they can stay in core as long as they're making progress towards being stable.

This means we definitely would not add an alpha (or even beta) stability module to 8.LAST.x. Because it has very little chance of getting to stable, and also because we'll need to concentrate on getting 9.0.x ready and ensuring 8.LAST.x is as stable as possible in relation to security issues and critical bugs.

But it gets trickier with experimental modules that are already in core by this point.

Two questions then:

When we open 8.LAST.x, what happens to beta stability modules - do we remove them and add them back to 9.0.x? And if so would we consider releasing 9.0.x with a beta or rc stability experimental module that's not in 8.LAST.x?

If not, when we tag 8.LAST.x, do we remove them (again?) and add them back to 9.1.x?

hmm I tend to think of that as the experimental modules themselves have bc promise (for convenience with testing etc.) but that core as a whole does not in relation to experimental modules (because they're experimental). i.e. if we really changed our mind on an experimental module, it could move back to contrib and that would be the 'upgrade path'.

If we got to this point, then the beta-stability module being in 9.0.x seems like it might be the best compromise? Because of the 1 year security support for minor releases, you still have 6 months to get onto 9.0.0 so there is continuity in available versions with the module included. But people using 8.LAST.x don't end up with an unmaintained branch of an experimental module.

This does mean we'd really want to minimise the number of modules this happens to, because it will be annoying for people using them regardless.

Added related to summary

I think the last time we discussed this, we thought that it would be more disruptive to remove a beta stability module in 8.9.x than to leave it in - also the requirements for beta-stability are a bit better defined now than they were when I opened this issue.

So something like:

1. No new beta-stability code added in 8.9.x
2. Existing beta-stability in 8.8.x can probably be left in 8.9.x, unless we have an additional reason to remove it from the branch.
3. Modules can up their stability during 8.9.x alpha/beta/rc stages same as they can for other minor releases.

I'm going to add this as a beta blocker for 9.0.x since that's when we normally have deadlines for experimental modules - but we may want an earlier beta release for 9.0.x which will also impact the beta date for 8.9.x too.

what about config_environment module? should this also be tracked here?

We agreed to allow beta modules to roll over between major versions. We'll repeat the evaluation process for Drupal 10 in #3118154: [meta] Deprecate dependencies, libraries, modules, and themes that will be removed from Drupal 10 core by 9.4.0-beta1.