And slavery doesn't add anything to Dark Sun. Everything else in Dark Sun makes the setting interesting and unique. So go on, name two things that slavery actually adds to Dark Sun that (a) would actually make Dark Sun totally different if removed and (b) can't be replicated with serfs or paid workers.
Adventure plots where the PCs are caught by slavers and have to escape.
Freeing slaves as a PC activity.
Dark Sun seems the best supported TSR/WotC setting for freeing slaves as a PC activity. There are evil slavers and slave holders in a lot of the setting. There are free areas with lots of motivated ex-slaves (Tyr and slave tribes), there are lots of ex-slave concepts and NPCs. There are mapped out trade routes of the slave trade you can raid.
Spelljammer has the Neogi as bad guy slavers you can target for freeing slaves. There are big illithid and beholder factions too who likely have slaves. But it is mostly a thing of the monsters and not really developed as a setting thing beyond that.
Forgotten Realms has the drow as stand out slavers, there is some Zhentarim slave trading activity, and I am sure if I looked deeply enough others. Mulhorand and Unther have the history of being historical Ancient Egyptian and Mesopotamian humans drawn to Faerun through gates to be slaves to an ancient magical power, so the nations and the associated pantheons being in Faerun are tied to slavery. It is not nearly as central a setting thing as Dark Sun though.
Greyhawk has drow too, but also the Scarlet Brotherhood and the whole against the slave lords set of modules.
Dragonlance has some slavery, including stuff like one subrace of non-drow elves enslaving another.
Eberron I can't think of much slavery off the top of my head, I'd have to do some research.
Mostly though I would suggest Dark Sun if you want your fantasy adventure to be hunting down slavers and freeing slaves.