The other issue this raises, for me, has to do with appropriate media. Should art explore controversial topics? Yes, I think it should! In fact, I think it is vitally important that it do so! But should all art explore controversial topics? Nah, you gotta pick your contexts. And I don't think D&D is a great venue for exploring complex themes around, say, slavery and eugenics (not that OG Dark Sun really explored those themes in a complex way - it just kind of lays them out there as part of the setting).
I 100% agree that while you want media and RPGs to be able to explore controversial topics, not all should. And I would say D&D does exist in this middle territory where it is the biggest game but also has a history that has been open to perhaps not controversial content as much as content that isn't strictly for a young audience (i.e. Ravneloft had elements of sensuality to its and some of its content to highlight the gothic themes---though all fairly tame; Dark Sun was a brutal post apocalyptic world with all that came with that genre, etc). I guess where I am coming from here is I don't necessarily think slavery needs to be that controversial in a game (especially when it is largely drawing on slavery that has existed in earlier periods than US History). I also think, while the core D&D game is meant to be pretty general and probably shouldn't be tackling controversy that much, the whole point of having settings is it creates niches where things like that can be addressed (and I would say Dark Sun was a perfect example of how a mainstream, fairly conservative company was able to put out something that had a lot of challenging themes in it).
Sure, there are some tables where such themes might be considered with the gravitas and contextual nuance they require. But there are also going to be a lot of tables where they don't, and then you can wind up with the optics of young players going, "Woohoo, I'm a racially superior slaver!" Most players don't come to a session to engage in thoughtful art analysis, is all.
"Woohoo, I'm a racially superior slaver" isn't really what Dark Sun is about though. If you read through the narrative of the gazetteer in the boxed set there is a lot of hostility directed at the powerful, like the Sorcerer Kings (who are destroying the world to aggrandize themselves). Yes, you could play a slaver and you could say that, because slavers and slaves exist in the setting, but I don't think that was at all the intention of it.
Also I am not convinced young people can't handle navigating these kinds of optics if you trust them to do so. I've had plenty of conversations with people younger than me who can handle a conversation like this one and how can understand there is a difference between the slavery in something like Dark Sun and the slavery that was used in the US (and also understand that the mere existence of slavery in a setting isn't necessarily a commentary on US racial politics or its history----that these were institutions that existed all over the world and throughout history).
Personally at my table, when it comes to stuff like this, I don't think it adds gravitas. I just think it is important to be able to draw on the range of experience in history, on the range of tropes in the source material to make for an exciting game. In the case of Dark Sun, I think there is a bonus that this also happens to be a setting that rises a little bit above to become something more meaningful (but I think that is rare).
And if tables don't want to handle any of this stuff, that is fine. I am not saying everyone must enjoy X! I just think we are setting the bar pretty low with all the things that have been getting taken off the table in these conversations. Keep in mind this is just one piece of a much larger discussion that includes removing or revising elements like evil orcs, tropes perceived to be colonialist like going into a dungeon, killing things and getting treasure (arguably the heart of what D&D is), etc. For me I am viewing this conversation in the context of all the conversations we've had about this, where I think there are a lot of people, who may be very well intentioned, leading us down a path that is not going to produce better games and even make good gaming near impossible to have.
These are the kinds of themes that are best left to indie games, for those who seek it out - the RPG equivalent of mature rated films.
I don't think Dark Sun is mature content though. I bought the boxed set in 1991, so I must have been like 14 or 15. It wasn't anything I couldn't have seen on TV during the day. More brutal than Dragonlance, but it isn't like it was the 1979 version of Caligula. It was content you would have encountered if you read any fantasy literature at that time.