After the Apicalipse there was a website where you could see which instance de/federates with others, as a map. It's url was https://lemmymap.feddit.de/ but I guess it went down with feddit.de I found even a screenshot in a thread:
Currently with the instances button you can see defederation from one direction, but not from the other, e.g. if an instance is defederated by a lot others you can't see that easily.
Does something like this exist?
It seems like. This is the main admin account from mastodon: link I can see the same posts directly: https://app.wafrn.net/blog/admin