Every highly curated subreddit seems to have a regression toward some sort of lowest common denominator and any sort of deviance gets crushed even if it's good correct content that shouldn't be. I'm no historian but seeing this behavior in every subreddit where I have the experience to recognize BS makes me very skeptical of the idea that other subreddits are not doing the same.

What's keeping it on Reddit, just inertia?

r/AskHistorians has some very good posts, but it also has a good deal of misinformation, even coming from flaired users. The big problem with it is that the user base and the mods tend to believe that it's accuracy is better than it actually is (and comments from the mods seem to encourage this overestimation). I've seen many people who believe that if a flaired user on r/AskHistorians says something, then it's true (there seems to be a lot of people like that on the aptly named r/badhistory sub, for example).

In addition, their yearly April Fools' events (a bad idea in my view) as well the generally poor quality of posts during the first few years makes searching for past answers a pain. You get a ton of information that's flat out wrong, and in the case of April Fools' events the wrong information was actually encouraged (or written) by mods.

It's indeed goldmine for anyone who's into history. I'm debating myself if this sub is good enough to keep Reddit around or am I better off to just stay completely away.

Log off and bookmark the specific reddit. It's only addictive if you use the home page features.

I was usually using the Reddit is Fun app, but yeah maybe a direct subreddit link in browser could be a good middle road.