Phpbb forums didn't go away because of self censorship, they were simply replaced with bigger sites like Reddit. If you're trying to create a community around a niche topic, a subreddit is substantially easier to create than deploying and maintaining a forum.
I was not aware that phpbb forums disappeared; I regularly read three of them that are all quite active. Yes, there are subreddit counterparts, but the reddit versions are mostly filled with crappy karma-seeking memes and wildly off-topic discussions.
The phpbb forums tend to be a lot more ruthless in self-moderation and probably a fair amount of gatekeeping too. But the content is significantly better than reddit, which I believe is a source of pride for many of the participants. I also get the impression that there is a growing sense of “coolness” using a crappy outdated UX. I predict the return and rise of the niche community sites will happen before its total disappearance.
