"All slaves must be set free" was exactly as true in 1750 and it is in 2022. This isn't a historical or sociological analysis; it's moral. If you built a time machine and traveled back to 1750, you could not morally entertain the then-popular norm of slavery.

I think there's actually a big difference between morality and truth, and what you're saying applies to truth and not morality. If you went around telling people about germ theory, you could say, I'm objectively right and you're objectively wrong, here's the proof. The truth is the truth; it's an invariant. It's arrived at through discovery and investigation. Morality is about people fighting to get what they want. The "right answer" is arrived at through conflict and seeing who wins. We live in a society where "slavery is bad" won.

When you say that we should ban debate of slavery because it's settled, what you're in effect saying is that society has made a decision and you would like that decision to be final. In this case, so would I. But there's nothing final about it; it's not like God has spoken on the matter. Deciding to ban further appeal of the matter is just a form of moral entrepreneurship, not some kind of absolute correct answer.

Slavery is not wrong simply because the union won.

I am sincerely curious: are you able to provide a criterion or two that would allow me to make objective moral decisions (the possibility of which I think you are postulating)? How do I know slavery is immoral, besides that it is considered immoral in the given cultural context? I am sure an Athenian gentleman 3rd century BC didn't think that way; Aristotle surely didn't - and he spent a lifetime on systematic reasoning, ethics incuded.