I hope this is the right place to discuss a potential feature for lemmy.
I've been reading a lot of the defederation calls from instances and their users. More often than not, this was due to very specific elements of those instances; trolls, extremists, etc... But in my opinion, defederating a whole instance because of that is a sad pity.
I was thinking a way to solve this would be to have a federated blacklist.
Instance Admins would ban user accounts from their instance and that would be added to a list that could be consulted/automatically used by other instance owners. They would ideally be able to set parameters, like banning users from a list accepted by a number of other instances, a specific reason for the ban, or banned by specific instances.
This would lessen the administrative load, protect instances, allow different instances with shared concerns to help each other while allowing their own users to interact with the 'compatible' users and communities from other instances.
Just an idea and wanted to bring it up and hear some thoughts.
You are correct.
The value is part of the person object so it just propagates naturally. Whether remote instances actually act on the information in any way other than populate the BANNED USERS list in the Admin UI, I don't know. As you pointed out, they can't login in the first place.