hexbear.net is "A leftist social platform centered around community building through discussion, shitposting memes, and sharing content." They are fairly large:
hexbear.net were not a federated instance until recently, when they announced their intent to start federating with a select group of other Lemmy servers.
lemmy.world claimed to see ulterior motives behind this intention and decided to pre-emptively block hexbear.net from being able to federate with them.
It's becoming increasingly apparent that the largest Lemmy instance thinks it has a duty to protect its users against anything that might cause upset, sometimes before it even happens.
From everything I've seen the lemmy.world moderators are pretty bad.