It includes in its Blocked Instances list, has defederated with, 181 instances.
Now, you might well agree with some of those being blocked. Like, maybe they're spammers or harassing people or God knows what. They might host speech that might be illegal in some jurisdictions, be classified as hate speech there. They might contain content that's socially-unacceptable in some countries -- one of my first experiences on the Threadiverse was being sent by a random kbin.social sidebar comment recommendation into a conversation that Ada, the lemmy.blahaj.zone instance admin, was having with some guy in the Middle East, whose country had apparently blocked that instance at the national firewall level due to it having LGBT content or something like that. There's pornography on lemmynsfw.com. Consentual-nonconsentual and synthetic child pornography on burggit.moe. Piracy material on lemmy.dbzer0.com. Some instances won't approve of that being accessible from their instances, and in those cases, the instance admin is already blocking things.
I chose my home instance -- lemmy.today -- specifically because it was an instance policy to try to avoid defederating with instances, and it presently has an empty blocklist. But as best I can tell, most instances have some level of content or user behavior or whatever on other instances that they consider unacceptable and will defederate over. Maybe not it's not Threads, but they're aiming to block something.