It's been a while but I don't recall a ton of politics on the usenet groups I used to participate in (mostly tech subjects). Of course there were groups for politics, and sometimes a thread would go off on a tangent, but the nice thing about usenet is it was easy to killfile people or threads that you didn't want to read anymore.
When I used it it seemed like it was mostly real names (.edu accounts) at least on the groups I read, so maybe that kept people in check a bit. Though that would have been easy to spoof I'm sure.
My department also had some local groups that did not propagate. So it must be possible to stand up your own NNTP server and have groups for your local users that are at least in that sense "private"
Usenet was sorted by topic. Same people stick to the required topic in different forums.In the fediverse, you follow people, and therefore can't control very well what people post about. I follow some people because I am interested in technical topics, but I end muting some of those because some are very political, and I am not interested in some of these topics.
I don't think the Usenet model can be replicated on top of Fediverse. Not everyone sees instances as a topic thing (I host my own instance).
Lemmy is also built on ActivityPub as part of the Fediverse and is arranged into topics (called communities). E.g. you can follow retrocomputing@lemmy.sdf.org (or web interface https://lemmy.sdf.org/c/retrocomputing) and only see retrocomputing posts.
Since it's all ActivityPub, you can follow Lemmy communities from a Mastodon client but the UX in that case is pretty bad (e.g. Mastodon servers won't backfill posts so you won't see any history).
When I used it it seemed like it was mostly real names (.edu accounts) at least on the groups I read, so maybe that kept people in check a bit. Though that would have been easy to spoof I'm sure.
My department also had some local groups that did not propagate. So it must be possible to stand up your own NNTP server and have groups for your local users that are at least in that sense "private"