It’s a weakly specified / trick question. Do you get to keep your knowledge of the world as it is today ? if not how are you different than people 200 years ago ? So you can just ask what your ancestor’s position would have been. Which we already know the answer to.

It's pretty clear that you don't get to keep your knowledge of the world as it is today - if you did, the question would collapse to "Do you support slavery?" which is much less interesting.

Asking what your ancestors believed is also a bit of a miss. My ancestors in the US were all German-American, and German-Americans overwhelmingly opposed slavery, so my ancestors probably opposed it, and I probably would have opposed it too if I had been part of that population, but that's obviously a total cop-out.

So take the framing of, say you were born into a white slave-owning family in the antebellum American South. Would you have freed your slaves and joined the abolitionist movement? The actual people in that situation weren't entirely a monolith - surely at least a few of them actually did that. But if the actual share who did so was, say, 1% of the population, then you have to think you're in the top 1% in terms of some combination of empathy, racial tolerance, forward-thinkingness, etc. to claim that you would. The question is useful because we're not different than people 200 years ago, no matter how much we like to think we are.

I think the obvious and more explicit follow-up question is even better though - "What beliefs do you hold today that will be viewed as negatively 200 years from now as slavery is today?" If the answer is "none," we must be in such a utopia to have finally reached the end-state of human moral development.

People don't want to admit that evil is done by people like ourselves. Asking what your dead evil ancestors would do is different from asking what you would do.

If you were born back then, with no knowledge of the future, you would be a Good Person no matter the conditions, right?

The question forced to me ask myself if my values and convictions remain strong in the absence of knowledge. Do I know that slavery is terrible or do I believe in equality of humans? In the first case it's memorization, which is very specific to context. If I believe in the equality of humans, I can apply that belief across contexts and still reach unconventional conclusions.

Personally, I know I would not have been much different because I'm not much different today.

It's an interesting question for a lot of reasons. I live in the South, so perhaps if I had been living in the South in 1859 I would have supported slavery. But my ancestors were living in the North, in Pennsylvania and Ohio, and fought in the Civil War on the side of the Union, so presumably they opposed slavery. So if I had been reared by those ancestors, I likely would have opposed slavery. But then again, I'm a bit of a contrarian by personality, so I may have had the opposite viewpoint from my surroundings. Perhaps not opposing slavery in the South, but opposing fighting to preserve it. Perhaps not supporting slavery in the North, but opposing a destructive war and supporting the rights of individual states to determine their own fate and secede. Or personal things could be involved. Suppose as a northerner I saw an opportunity to join the family of a wealthy Southern planter. Or as a southerner, I wished that certain slaves who were friends could be free. So it goes down to what your environment is and what person you are in that environment.