> Killfiles didn’t seem to work.
What I used to do was to killfile crossposts to certain groups, the trolls themselves, and subthreads started by the trolls. It worked reasonably well at the time.
Usenet, much like SMTP (but arguably unlike XMPP) built enough momentum before the abuse exploded that there's no reason it can't continue in perpetuity, even if individual groups might come and go. But unlike SMTP it's much more at risk of collapse if good people walk away unnecessarily.
Usenet's not really a single entity. I used it fairly heavily from about 1997-2005; most of the groups I read then are now either tumbleweed or overrun by spam. But a couple of them are still running along happily (if a little smaller than they were). So I think 'still healthy' is pretty contextually dependent.
Killfiles didn’t seem to work. Even if you blocked a crackpot, one of the non-crackpots would still stupidly respond to him – a lot of people never understood “Don’t feed the trolls”. So you’d click on a post only to find out the discussion was people pointlessly trying to argue with the crackpot. The crackpot’s own posts might have been hidden, but everyone just quoted his posts in their own replies anyway.
A few years ago, after someone somewhere claimed, like you, that Usenet is still healthy, I had a look back at the group to see what it was like: still dominated by those same crackpots.