That would certainly be one way to handle it, but it brings up a few issues to my mind.
One, like I brought up in my comment, it would be pretty contradictory to a decentralized platform like Lemmy/the Fediverse. Every instance is run the way the admins wish, and having a forced banlistwould be pretty contrary to that idea. If a central authority controls the platform, it isn’t very decentralized, is it? That said, even if we accept an enforced banlist, how effective can it be?
It would need to be handled by a person or group beyond reproach, there would need to be an ironclad way of telling which instances are homes to bots, and it would need to be constantly maintained to add instances as they were found out. None of these really translate to the real world, unfortunately. And even if we get lucky on all of those points and it worked out for a while, introducing a way to block instances off from the entire platform without approval is a pretty big risk if it ever falls into problematic hands down the road.
And if it’s not enforced, we’re left relying on all the instances agreeing, which is just not going to happen. Some instances will decline to work together out of principle, disagreement, or just contrarianism. And then we have all the “dark” instances that are left unmaintained and updated. I’m not sure how much of a problem that latter group would be, overall, but I figure it would lead to some issue or another. Maybe I’m over estimating the effect non-participants would have, but even if that’s not such an issue, what happens when big instances have disagreements, or start their own banlist? Then it’s just a fractured mess that isn’t really helping anybody, doing more to hinder efforts against bot havens than it is helping.
All in all, I just don’t see a good way of it working. I know I’m not really offering solutions here. I’m really just poking holes everywhere, but that’s kind of my point. I hope I’m wrong and there’s a way to address this that I just don’t see. I really like this whole decentralized thing and I want it to work out!