In terms of Lemmy instances, assuming the feddit.de map is up to date, only beehaw, tucson.social and toons.zone have defederated. The first two presumably because of open signups.
However, afaik, Kbin doesn't publish it's blocklist, and the timing does make sense (last posts seem to be about 2-3 days ago), so it's possible they have blocked it.
On the other hand, kbin.social had pretty much always had issues federating since the "migration", so it could just be under load.
From a technical standpoint, there is no real difference, it comes down to how the instance owner feels it's best to run the server.
Ultimately, instances (or at least the ones most people want to join) want to keep rulebreakers, trolls and spam out. There are two main ways of doing this:
Of course, there is a lot of debate as to which of these methods are better (beehaw, for example, fundamentally doesn't think a reactive approach can work at all), which causes tension between some instances.
This tension can rise to a point where one instance "defederates" from another, meaning they stop talking to each other and you can't interact with one if you have an account with the other.