https://meta.wikimedia.org/wiki/Community_Tech/Cross-wiki_watchlist says that "Technical work on this project is complete" (and also "Development will continue into the beginning of 2017", could that be updated?).

Done.

Is this maybe superseded by MediaWiki-extensions-GlobalWatchlist in 2021? Should the Phab task be archived?

Yes. I've declined all open tasks except T5525 which is sort of the long-term epic. That can be closed once MediaWiki-extensions-GlobalWatchlist is refined and considered "complete".

The MediaWiki-CrossWikiWatchlist project itself may be archived. Keeping it around will probably only lead to confusion.

Thanks a lot!