As I said before (or possibly in another one of these Dark Sun threads), a quick google ("dark sun slavery legal") had the
very first result being a post on reddit about a party who bought a slave, and the DM wondering if the slave should come to love the PC owner as a brother. That post was from last year.
You can go on Reddit and find enough horrific stories of both players and GMs to spend days reading and weeks lying awake in bed with no hope left in humanity. None of them need a particular setting or setting element to inform their awfulness.
Some people will be awful, and being afraid of a topic because somebody can be awful with it does not mean that removing the element will make the people stop being awful. It just means they'll be awful about something else.
As for as my own feelings on Dark Sun, while I read a number of the novels and found them interesting, I never had any interest in playing the setting. Part of that was that I really disliked the D&D system at the time* (5E got me to like it again), and part was that playing in grimdark settings (for example, White Wolf's edgier cousin, Black Dog) doesn't really appeal to me.
* Note: I liked the D&D
worlds — I read a huge number of the various Forgotten Realms/Dragonlance/Dark Sun/etc novels — I just didn't like the game system.
However just because it doesn't appeal to me doesn't mean I think the very concepts should be suppressed and censored. People like lots of things that I don't care for. I like things my friends don't care for. If something was particularly offensive to someone I knew, I'd avoid bringing it up, but the only thing I'd actively argue down are actual lies, and maybe stupidity.
Dark topics have their place, and oftimes a very important place for certain people in certain points in their lives. For example, see Neon Genesis Evangelion, which had an element which could be perceived as slavery, but which had a deep and meaningful impact on many people's lives, my own included.
As for the poll question: Just because something may seem awful to experience
now does not mean it will not be a learning experience for later in life. Maybe the GM learns that being permissive and letting the players run roughshod needs to be curbed, and he needs to hold a stricter moral stance on what is allowed, which in turn influences how he acts in non-gaming life. Maybe the adventure is so awful that it ends up on one of those Reddit threads, as an object lesson to those who will listen. Maybe an edgy teenager makes an ass of himself today, and deeply regrets it 10 years from now, putting a brake on behavior that he can now recognize as awful
because he had to go through that with a gaming group and it blew up in his face, rather than suffering from subtle and silent cold shoulders he never recognized.
I would even say that conditioning inclusion of "problematic" content on making it obviously evil, and having an obvious way to fight it, misses the mark. Players can find a way to fight against literally anything. It's not about being told, "That's evil!", as petty moralists love to do. It's about learning about the impact of various elements of the world, and the
player learning how to deal with them.
Hiding from a thing does not make the thing go away. It just makes it so that people don't learn how to cope with it, or how to deal with it, or how to talk about it. This is
especially true of stories, one of whose main values is teaching us lessons in ways that don't get us killed by forcing us to learn those lessons in actual life experiences.
It may be literally impossible to end slavery, especially in a world where it's pervasive. (It's not like it's extinct even in the real world.) But that doesn't mean you can't do
anything. You can help individuals. You can hide families from slavers. You can save towns from raiders. You can help slaves escape. You can lobby (bribe) government officials to make lives easier.
Your characters may want to become as gods, but even gods can't do everything. The value of such dark worlds is learning that even in the face of utter hopelessness, you can still do
something. And that's a lesson that translates to the real world far better than being able to defeat anyone who does something you don't like (which sounds like a lesson which gives rise to Karens).
Now, whether a particular company wants to be in the business of publishing such a dark world is another question entirely. Their business is about making money, not fighting against anyone who gets offended by the existence of a concept. Maybe a company like WOTC would approach it by making a second brand for its "dark" offerings (similar to White Wolf/Black Dog). Or maybe they just leave it for others to explore.
Their choice is their choice, and really has nothing to do with the abstract question asked in this thread. The only "problem" would be if they lock the IP up so that no one else can use it ever again, but that's a copyright problem, not a moral problem.