Well you have just described Metafilter. I’m a liberal a lefty as can be, and eventually even I got tired of the drama and obvious virtue signaling. And at the end of the day, drama and less-than-appropriate virtue signaling were what the mods wanted.
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.