I don't see much/any negativity towards games that include stuff like slavery as a setting element, but the issue is whether they make it kind of a central issue, and make it more like Roman-style slavery, or more like Southern-style slavery. Unfortunately, despite the presence of some Roman and Ancient World trappings in Dark Sun, the slavery was more oriented towards the Southern-style, replete with an entire race of beings who only exist via slave-breeding (and who unfortunately play into some weird racist tropes about human breeding associated with slavery).