I can't really speak to general interest. As I said, discoverability is terrible, so I only know of stuff that either I've been following since the '90s or I've seen mentioned by others. So this is a very eclectic list. But they all have recent discussions going on.
If you know the usenet hierarchy, you can enter the start of a set of groups (e.g. "rec.games") and the current Google Groups search interface will pop up a list of Usenet subgroups after a sec...but only if you're not already within a group. But the results look to be roughly sorted by recent activity, so that's at least helpful in revealing which of the groups are pretty much dead. (Undead spam threads are pretty common too, unfortunately.)
Note that Usenet quality has always varied (it's the place that more-or-less introduced trolls, viruses, spam, and flame wars). I really can't tell if the quality of the posting has gone up or down. I definitely can't guarantee what you'll find.
Note that Usenet quality has always varied (it's the place that more-or-less introduced trolls, viruses, spam, and flame wars). I really can't tell if the quality of the posting has gone up or down. I definitely can't guarantee what you'll find.