Hmm. Is that the problem with including slavery, including chattel slavery of entire races, in your dnd game? That it's too "metal," or R-rated, or "not safe for work"? That including this theme can be there to "test your limits"?
This is similar to the discussion of the "grimdark" Goblin Slayer game. Including gratuitous violence, especially violence that has resonance with people's histories and life experience in the real world, does not, IMO, make a game more adult or mature or serious. Many grimdark settings strike me as in fact childish in the way they present exaggerated and graphic violence as an attempted means to build narrative stakes, and I think it has the effect of actually trivializing the impact and trauma both within the setting and, more importantly, for players at the table.