Closed, propiatary, with separate ux/client for every silo? They are not usenet in any meaningful way.
They're more like bbs forums in a world were there was no improvements made, and the internet never happened.
The only thing (most) have, is that for traffic purposes they allow spiders like Google and Bing to enable federated search.
But there are variants like Instagram that allow search, but force login - and things like Facebook groups, that are opaque.
The ux of most forums (including hn) is terrible. HNs one saving grâce is that it's simple (as it should be).
I don't mean to jump on you; I get that things like reddit is a place were all people can exchange messages - but I don't think that makes it modern usenet. Just like mailing lists were/are not usenet.
The defining characteristics of Usenet was not the license or client (IMHO the UX of most NNTP clients was pretty terrible for casual users) but the content. There was a group for everything that anybody could access as long as their ISP provided access. If you asked me if there was some place on the internet that had a discussion group for everything I'd probably say Reddit is the closest thing. All you need is a connection to the internet and a browser and you can start reading. Posting requires an account but they are free.
I agree with you that forums, BBS message boards and mailing lists are nothing like Usenet.
Threaded discussion is best done simple, and that is what we have.
That said, there's a couple of decades of horrible web platforms from nuke/phpbb thorough reddit and Twitter. It's not exactly high price to be "not as stupid" as all the the ones taking pride in their convoluted stupidity...
What we have here is dead simple, works great, very lean, effective.
All excellent.
We could have more and that may or may not trend toward excellent.
The clear problem with that is the lack of federation, but I don't think users generally care about that.