Each instance has a publicly visible lists of “Linked Instances” and “Blocked” Instances. I’m curious to see a “Blocked By” list showing which instances have blocked a particular instance. Is that information readily available anywhere?
If possible, it would be interesting to have this information on an instance’s Instances page. The attached picture is an example I cooked up.
At most you could probably list instances that the current instance tried to federate with but failed. Since instances only try to federate with instances where users have a subscription to a community on the other instance there might be instances out there that just never even tried to federate with a given instance that still have it blocked.