There is no ActivityPub category on Meta; there is a tag for that. And it’s the tag that follows the category on your forum. So topics here are created uncategorized but with the activitypub tag.
Angus explained that before in April:
A community member moved the post to the support category. I guess I would have chosen the same category because, to me
sounds like they need help (in contrast to, e.g., reporting a Bug or requesting a Feature).
Though the General category, where it is now, of course, also works, as general works for everything.