Okay. That’s you’re opinion. There are lots of really bad things that happen in games. Genocide, invasion, human sacrifice, piracy, murder, mass starvation, and world ending destruction.
I believe that Muls need a re-work. The name is far too close to the dated and offensive and should just be replaced with half-dwarf just like half elf and half giant.
In principle though I see no problem being able to explore issues of slavery in a game context. Provided the game isn’t encouraging or enabling that behaviour. Odyssey of the Dragonlords had Minotaur slave gladiators

The PCs are strongly encouraged to free at least one of them.