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?
LOL, at a quick glance one might even be able abuse a scheduled job on free-tier GitHub runner for the data update and host it statically on GitHub pages.
"Serverless for the poor"
and you will do it?