Larger ones I don't mind mentioning:
I think bodybuilding.com is still what it was back in the day.
Sherdog is probably also still decent though I don't follow it.
The surviving /chans capture a lot of the spirit of old timey Usenet as well for better and worse.
If you're into nice wristwatches watchuseek is same as ever; amazing the energy that goes into such a niche thing.
Men's clothing/accessory forums are also quite active and good; askandyonclothes, styleforum and ... tho I haven't checked lately fedoralounge used to be good. These are kind of remarkable in that they, like the watch forums, could be monetized in various scumb bag ways, but somehow remain good communities oriented around consumer goods.
I think lot of energy still goes into old school email lists for special interests as well. Though I don't follow it as I should, the J lists are extremely high bandwidth/quality. IMO these are vastly better than platform things on discord, reddit, discord, as they're always backed up and uncensorable. You can get kicked out, but you'll still be able to read most of them.