Communities can eventually become insular and crappy, that isn't anything new. I haven't ever used/heard of metafilter , but I believe you.
Not a problem unique to lefties or hardcore MAGA folks. It's just community management for free by volunteers eventually means you have some echo chambering. The site/community manager can steer the mod policies, but without leadership you get fiefdoms. Look at some subreddits that speed run this process.