https://lemmymap.feddit.de/

I think you can see how many instances an instance has defederated from here, in the “Bl” column https://github.com/maltfield/awesome-lemmy-instances. scroll down to All Lemmy Instances

You can check for any instance who they have federated with and who they have blocked by navigating to their /instances page. For example for Beehaw: https://beehaw.org/instances

I’m not sure if it’s exactly the same thing, but you can click on Instances at the very bottom of the instance, or add “/instances” to the url e.g. https://lemmy.ml/instances to see the list of blocked instances.