Hi @Jenkler! Long time no see 😁
I don't think we can do this right now, since visibility is handled on the category level.
It could likely be done in a plugin though, to enforce visibility on a per topic level ..
When you go to /recent
Without being logged in you can see the titles to posts, see the avatars and mouseover to get the names. Same thing in /unread (well, you have to be logged in for unread but you'll see post titles in categories that you dont have read access to).
Also you'll be notified if someone @ mentions someone in a private forum.