Watchlist Expiry[1] is partly merged and currently in an incomplete state. There are many patches[2] already in master and taking those out is likely not a small chore.
This task is here to re-evaluate the state of Watchlist Expiry before the release and either advertise it on the RELEASE-NOTES as ready or to minimize exposure and mark as experimental.
[1] https://meta.wikimedia.org/wiki/Community_Tech/Watchlist_Expiry
[2] https://gerrit.wikimedia.org/r/q/(author:hmonroy%2540wikimedia.org+OR+author:musikanimal%2540gmail.com+OR+author:sam%2540samwilson.id.au+OR+author:scardenasmolinar%2540wikimedia.org)+project:mediawiki/core+status:merged+watchlist