I wouldn't say defederation is rampant, but there were a few very high profile examples that hit right when people were new. Specifically, Beehaw/LW and Exploding Heads/everyone. Plus the whole thing about NSFW content and types.
While it's true that many of the original devs have problematic views, it's not really meaningful. The software is open source, there are tons of new developers (with varying views), and the code has nothing to perpetuate their ideas. In fact, they are pretty isolated on their own instance (Lemmygrad) since everyone defederates them almost immediately.
I've been hearing different answers on this lately, particularly regarding Lemmy.world and Beehaw. Beehaw defederated LW, but not vice versa. The result is that no one on Beehaw can see any posts on LW, but users on LW are seeing some posts on Beehaw. It doesn't seem to be a complete cut to all communication.
But that's still something worth bringing up - Here is the list of instances that block FMHY, and are (somewhat) effectively isolated from us here:
https://fba.ryona.agency/?domain=lemmy.fmhy.ml
The only one that's significant is feddit.de. From what I can tell, it's because FMHY has open registration