Lemmy

Given that I asked the question in the first place, here is how I make sense of it after some answers and looking at the modlog.

What I don’t understand is why does server B have to tell server A that user@B is banned on B, instead of just not letting user@B login anymore and thus disallowing the creation of further content by user@B?

Because A sees B's content, but on A server. user@B is banned on B, hence, A keeps seeing B content, but not user@B content on B. user@B can still register on A and post there.

And maybe A wants the content by user@B, so why can server B dictate bans that server A has to enforce?

Because user@B's content, while copied and displayed on A, is still B's content. B bans the user and their content everywhere. Again, user@B can register elsewhere and start posting.

What happens if server A ignores them?

From what I can see, if I am A, I cannot unban user@B in any way. user@B can register in my instance, though. I can ban user@A afterwards (or not).

As an instance would I be able to flood another instance’s ban list by just pretending to have trillions of users that are all banned?

I think so. I federate with Beehaw. If Beehaw bans everyone, my banned user list will get quite long.