Could someone clearly and succinctly present an argument against having slavery in a setting in such a way that their argument isn't an appeal to emotion? Because I really don't see the issue. Yes, slavery is bad. But I have no problem with it being in a setting, or for that matter the players owning slaves or enslaving people, or the PCs potentially being enslaved. If you care about simulating a plausible world, these are things that very well could happen. It would likely occur for much the same reasons it occurs in the real world, and would thus be thought provoking (at least, I certainly hope it would!). I also think people are focusing far too much on the transatlantic slave trade and completely ignoring other instances of slavery that have existed, in particular the ancient world and the Islamic world (as an aside, it was fairly common practice in Muslim empires to castrate male slaves (I believe this also occurred in China, and too a lesser extent Rome)). Excising slavery from a setting purely on the basis that it is bad, and claiming that it is a lazy/cheap tool to use when building a world is dismissive of people in the real world that managed to achieve remarkable things despite having been enslaved.