frankm@linux-nerds.org this is now done. Turns out I wasn't actually stopping a category from following a user, so that didn't need rolling back.
https://github.com/NodeBB/NodeBB/issues/13232
hamiller@forum.wedistribute.org using a hashtag is probably also something I should allow categories to follow, although that's local-only to the instance because of how hashtags work.
Either way I didn't want to restrict a valid use case :)