Very tanky ultra left pro Russia, pro China, anti everyone else part of Fediverse.

Check them out if you like but they only talk shit about the West and NATO. Put another way, you are not missing much by avoiding them.

Because basically, everyone got scared from them being communists/socialists and just decided to defederate from them.

It's nothing more than fear based on years of propaganda pushed by western countries, nothing more. They don't defederate from anyone, yet everyone feels the need to defederate from them. Yes, they do have leftist views, they do think the world of many current and ex communist leaders, but if you don't like that, you can just block their communities 🤷.

Other than that, their communites are mostly like everyone else's, politics, memes, piracy, comics, etc. They even have a few LGBTQ+ dedicated communities and about 20% of them are LGBTQ+ acording to a recent demographic survey they had (you can find it in their main community), so... basically, they're human, just like the rest of us.

Many of them are well informed, not to mention highly educated, so I can see why there is fear amongst other instances - a debate starts, most people will flop regarding info, facts, whatever, they'll have the upper hand in the debate, so why actually try and listen to what they've got to say, they're just tankies anyway - defederate 🤷.

If you don't like their communities, just block them, no need to defederate from them... at least I can't see a reason.

Is there a place to see what's blocked, and why?

It's full of tankies (fascists who like the color red).

I'm gonna give you the benefit of the doubt and assume you're a second language speaker where "user" is a masculine noun, but other genders exist lol. I encourage you to use "themselves" instead.