yeah, segregation and discriminations are definitely different than slavery. I sometimes think the instinct to automatically emote at it as bad without any real thought drives a similar instinct in more leftist circles to emote negatively at nationalism and nations, which are basically institutionalized segregation and discrimination. Germany exists to keep the non-germans out (or maybe now to keep the germans in).
Ireland as a State exists explicitly to keep the English out of Ireland. But, despite the State of Ireland and color codes basically both existing to keep people the creators of those institutions didn't like out, people are very uncomfortable with that blurred line and want a hard break between them, which I'm not really sure exists.
But, if we must talk about slavery, from my current position of ambiliance on many topics (as I said earlier, a disillusioned librarian who hasn't really settled on a new set code), slavery becomes a bit less morally clear when one salami slices to it.
For example, lets say I voluntarily sign myself into slavery, say for a set period of 20 years in exchange for some non-monetary guarantees. Did I commit some grave moral sin? Did the person I signed the contract with commit a grave moral sin by agreeing to such a contract?
Or, to come at it from another scale, Country A sells a plot of land to country B. Implicit in the sale is the transfer of 100,000 people who lived on that land from one country to another. Did either country commit some great sin by selling these 100,000 people between themselves as part of the treaty?
To dig down a little closer to the Antibellum, when a peasant has his obligation set down between his family and the lords family, and those obligations stretch between generation, is a great moral harm done?
Now, I do agree that starting from the assumption that slavery is bad is not a bad knee jerk initial reaction (when I hear that slavery is come back in Lybia, I feel quite comfortable with my default assumption that some sort of injustice is going on over there), but I'm not really sure it deserves its current status as THE great evil, rather than simply being "a" evil. It seems to short circuit the ability to have a reasoned, calm discussion on a topic.
Hell, even a couple of years ago I remember being able to have a fairly calm conversation with, I think his name was something like firefox? over at spacebattles over this, and these kinds of talks, like how harsh of a contract could one sign up to and still be okay within a liberarian framework, or how multi generation commitments could work, were interesting points of discussion with my fellow libertarians in college! Slavery and intergenerational compacts are actually quite interesting intellectual exercises! That we have great trouble engaging with either is a shame.