Slavery still thrives in Thedas, even if the trade has been outlawed. Who hasn't heard the tales of poverty-stricken elves lured into ships by the prospect of well-paying jobs in Antiva, only to find themselves clapped in leg-irons once at sea? And humans fall prey to this, too.
If they're lucky, they end up in Orlais, which has only "servants." Most nobles treat them decently because they are afraid of admitting the truth. Orlesians go to great lengths to maintain the fiction that slavery is illegal.
Of course, the greatest consumer of slave labor is the Tevinter Imperium, which would surely crumble if not for the endless supply of slaves from all over the continent. There, they are meat, chattel. They are beaten, used as fodder in the endless war against the Qunari, and even serve as components in dark magic rituals.