Sometimes it's not even feasible to do the "When it is not possible for the new version to coexist with the old", e.g. when the break is because of a security issue or when something fundamental changes like the error codes.
We may want to add some text leaving us an out for these cases.
I've updated the doc with a third deprecation policy: APaskulin (WMF)/API change policy notes Let me know if we should adjust the language or include additional information.
Typically they're not. We usually reserve the mediawiki-api-announce mailing list for breaking changes.
Occasionally we might announce a new feature we really want people to start using, but that's the exception rather than the rule.
Good to know! Updates made here: APaskulin (WMF)/API change policy notes
fluorine was replaced by mwlog1001 a while back.
For MW developers, it might be more useful to just say they're logged to the api-feature-usage log channel. For client developers on Wikimedia sites, it would be more useful to point them to Special:ApiFeatureUsage.