As for it's "not considered controversial", buddy, it's on the way there. Just look at discussions in the same places you strongest objections to slavery being even portrayed in TTRPGs. I'm not saying "Hide your kids, the Federal Government WotC is coming to take your longsword D&D manual" or something. I'm saying the same sort of concerns that have rendered slavery not even acceptable as a background element are actually beginning to manifest re: murderhobo-ery and the generally violent and acquisitive aspects of certain TTRPGs.
Violence in RPGs (especially games where there are attempts to balance combat) is between people on mostly equal footing, while slavery is
not; nor is torture or rape. Those are the strong and powerful against the weak and helpless.
But anyway. Maybe RPGs will turn towards a less violent route, where players will first be rewarded for finding non-violent solutions, and then the games themselves will be written to not have violent solutions be feasible, or where violence is only considered acceptable towards mindless or irredeemable foes. Who knows? I got a
lot of tiny indie games from itchi.o bundles that were this exactly, and I know of (and own) a couple of RPGs that de-emphasize combat in favor of interpersonal relations (for instance, Thirsty Sword Lesbians) or investigation/puzzle-solving (such as anything Cthulhu based), so maybe the mainstream games will follow that path eventually. Maybe, in some point in the future, D&D will award XP based on how many foes you leave alive instead of how many you kill.
We're not entirely there yet. We
may be there one day, but not today. But we're
starting with the slavery, because at the moment, violence is still useful in games while slavery
isn't.
To be honest, I don't think anyone who wasn't already a sicko is going to think "Yay lets be slavers!". I very much think, based on long experiences of RPGs, that people who are messed-up like that, are messed-up regardless of the actual setting elements, unless the setting elements directly encourage them to be messed-up in like "It's okay to enslave goblins because they're not people!" kind of way - even then it only gets a few more people who were already on the edge of that.
If something is outright allowed in a game, then it won't be sick to engage in it. After all, as you just said above, we're often murderhoboes, and
that's "sicko" behavior. It's messed up to want to go forth and kill people and take their stuff, even if there's a pretense of "they're evil." It's why so many people were adverse to removing alignments in D&D--because they wanted evil creatures to fight, not creatures who may or may not be evil. It's doubly messed up to inflict burning wounds with fire and acid and lightning or to eat away at their souls with necrotic damage. It's horrific to take over a creature's mind and body with enchantment magic. And yet, neither you nor I would call the average gamer to be a sicko, because that sort of behavior is allowed by the game and most of us handwave the damage so it doesn't cause undue physical or mental pain.
I don't think
most people are going to say "yay, let's be slavers." I think people are going to end up with captives and, while figuring out what to do with them, realize that there's a slave market next town over and decide to get some coin for their trouble. Or they're going to need someone to carry their stuff and a slave turns out to be cheaper than a henchman. And if you have a setting where slavery is an accepted cultural norm, then yes, there
will be people who decide to play slavers because it's a realistic option or them.
I also feel like Star Wars really has a reckoning coming on droids lol though.
You ever read the webcomic Darths & Droids? There's definitely droid tension there.