It's than just slavery. That's what most D&D fans aren't getting.
- Slavery
- Racial supremacy
- Racial subjugation
- Sexism
- Rape
- Eugenics
- Cannibalism
- Ecological destruction
- Political corruption
- Magical Corruption
- Mental intrusion
It's a lot. Doable but it's a lot.
Of these, only 1 and 8 are present in Dark Sun more than any other setting, or aren't founded in lore that was mentioned in passing in one or two sourcebooks in 1994 and could be removed without anyone noticing. Slavery has been basically discussed to death here (and I don't think I can meaningfully add anything new to that argument at this point), and frankly I think that anyone who believes that ecological destruction as portrayed in DS would be a hot-button controversial political issue that would in any way give WotC second thoughts about publishing DS is heavily overreacting and inventing problems that aren't there.
2 - it's certainly in the history of the setting, if you delve that deeply into the lore of the Cleansing Wars etc in your game, but it's history only. Albeorn Elf-Slayer stopped trying to genocide elves once Rajaat got locked away. There are still elves. There are still dwarves despite Borys Dwarf-Butcher being the most powerful creature in the world for the last thousand years. Same with many other the sorcerer-kings.
3 - i assume you're talking about the prevalence of mul and half-giant slaves here? That's easily removable, even if you keep slavery at all. There's some slave half-giants and some free - just like there always has been since the very first DS boxed set.
4 - where? If sexism is a major part of the setting I must have missed it. But again, if I missed something obvious, the template for handling it is right there in SotDQ, which just casually retconned the gender requirements for joining the Knights of Solamnia, and pretty much nobody minded.
5 - the mul breeding thing? Simply remove it. Retcon it completely. Muls are the result of a union between human and dwarf, nothing more complicated or sinister with that. It was never important to DS, and nobody will miss it. This is the easiest of possible fixes and will be utterly uncontroversial.
6 - the half-giant and mul breeding programs I assume? Again, simply removed them. They added nothing of value to the setting, and nobody will miss them. For muls, see above, for half-giants, just say that the sorcerer-kings created them by applying enchantments to humans to accelerate their growth. Or else they always existed, like goliaths in other worlds. The eugenics was never important to the setting and won't be missed by anyone.
7 - yeah, just remove it, other than in necessary and specific cases like ghouls. Once again, it's a minor detail with no structural importance to the setting, and if its gone, nobody would notice. Maybe the halflings kill you so they can bury your corpse in the Ringing Mountains so you'll fertilise the soil and help expand the forest.
9 - come on, this is in every setting
10 - this too
11 - and this is all over the PHB and every D&D game and setting ever.
(You did however neglect to mention the question of PC templars, which given it makes your PC a member of the brutal enforcement regime of a tyrannical state rules by a genocidal monster, probably ain't gonna fly in the modern day. Just see what sort of reaction Paizo got to Agents of Edgewatch during the BLM protests! This is a much more legitimately thorny issue to handle than most of the above. But again, simply deciding that all templars are NPCs across the board would be a perfectly functional , if rather crude and blunt-force, solution.)
Honestly, a lot of the 'problematic' elements that are being raised here aren't Dark Sun specific, they were present to greater or lesser degree in every setting TSR created, and D&D has largely grappled with the portrayal of them in previous products. A lot of this post seems to be making mountains out of molehills. There are legit difficulties in adapting the DS setting to modern tastes. I'm certainly not denying that (though I still think adapting the system would be harder), but I don't think we need to overstate the problems quite this wildly.