In practice, all WMF-deployed code except MLEB and specialist fundraising ones run with branch compatibility, for performance and sanity as we remove deprecated code paths.
Wikibase is https://www.mediawiki.org/wiki/Extension:Wikibase_Repository and https://www.mediawiki.org/wiki/Extension:Wikibase_Client with both had rel compatibility policy.
I've corrected MobileApp, ExtensionDistributor, CodeEditor, CentralNotice, and CirrusSearch.
I've set the policy on extensions from \d to E, and for the six MLEB extensions (half already had it). More to come when I'm less bored.
(I note in general that we really need to update many of these.)