I’ve been reading many topics on this issue, and have a use case where the ability to exclude a group from access (e.g. to a category) would be most useful, in this fashion:
The use case is a Discourse instance dedicated to a fan community (for a multi-book & multi-season TV production) where ‘spoilers’ are an issue; members would like the ability to opt-out of seeing topics belonging to categories that are likely to contain spoilers.