I couldn't find any tools to check this, so I built one myself.
This is a little site I built: the Defederation Investigator defed.xyz. With it, you can get a comprehensive view of which instances have blocked yours, as well as which ones you are federated with.
The tool is open source and available on GitHub. Hopefully someone will find it useful, enjoy.
Its an echochamber that defederated whit most instances, and they very good at gatekeeping since if you try to sign up in there they ask you for an esay to sign up like if they where hot shit and theres a 30% chance they acctually let you in, and the moderation in there is pretty ban heavy and the admins are pretty much reddit mods that love to keep their echochambers closed. So thats why.
Those things affect other non consenting people. If someone voluntarily joins an instance with mod policies you dislike a) it's their choice b) they are free to leave whenever and move to a different instance
A lot of people on these large instances dont even know they've been defederating from other instances. I have yet to see any instance that actually votes on whether or not they should defederate and it seems wholly decided by the admins of said instance.
Otherwise thats what im doing, and i just complain about it cuz i can and dont agree with their methods. Im not ddosing them or trolling in there. They do them and continue to suck at it imho. And besides i just answered cuz someone else asked me why i think it sucks.