Hi there,
I created a Request for comments[1] and a corresponding Phabricator Task[2] regarding a feature that enables users to watch categories for page additions and removals.
If you are interested in this topic or somehow involved in Extension:Echo or watchlist features, feel free to participate in the discussion.
Cheers, Kai
[1]: https://www.mediawiki.org/wiki/Requests_for_comment/Watch_Categorylinks [2]: https://phabricator.wikimedia.org/T94414