Thinking about it. There are some things which can be done to better sooth the private forums.

Like to me especially signing up to each and every forum and then waiting to be accepted by a person feels good but has tons of friction and has some stress attached because you never know how strict the community is as well, it might take a day or two, perhaps this is the reason why we got the dumpster fire of mega internet forums called reddit or twitter of sorts

To me, federation feels better in this context since I can still have a single identity of sorts across multiple forums and you got better idea / ways to filter as well if need be

Another thing I feel about private forums where users have to wait for permission signing up is that I feel like something even as simple as having a cute cat or cute apple LOL or anything relaxing could make it less stressful for people to join. I assume its impact would be few but it would leave a deeper impact on those who do want to join.

not essentially private per se, but usenet groups and bbs had a natural vetting because it required some competency to even access it