The issue isn't whether slavery is good or bad.
That's an overly simplistic understanding.
The issue was that Dark Sun treated slavery largely as chattel slavery, a particularly bizarre and hideous form of slavery that is not actually that common in human history, but was practiced until quite recently by colonial civilizations in the Americas (particularly the USA and Brazil). In the US countless people have known ancestors (some with pictures and stories and so on!) who literally were slaves. I mean, the last person to die who came to the US on a slave ship died in 1940! She could have lived into the 1960s, she was only 75! This is pretty recent history and it's messed-up.
So the issue is, probably we shouldn't use chattel slavery particularly, and we shouldn't use "slave breeding" and the like as a concept. This is bizarre creepy stuff that's really from the US and from relatively recent history. If you look around the world, slavery has been common - serfdom and indentured servitude are, essentially, forms of slavery, they're just "limited" forms. Even the Spartans, horrible monsters that they were, practiced something which seems closer to serfdom with the Helots (just with bonus murder) than chattel slavery. The Romans also didn't really do chattel slavery, they did their own weird take on slavery which often ended up closer to long-term indentured servitude. Indentured servitude, as I noted, has been extremely common and perhaps most people, if they trace back far enough, will have people who were indentured servitude of some variety.
So it behooves Dark Sun to not use a weird, modern-ish American kind of slavery, when they could have something more apt, more historical, and less grotesque, and also less using the horror of people's great-great-grandparents and the like for entertainment value. That's kind of justifiable if you actually want to talk about US slavery, but it's a lot less justifiable if you just want to use slavery as a setting element.
And given how clunky WotC have been, and how tone deaf, it would be good if they could take some care here.