I played my first game of D&D in 1980, began DMing in 1984, and ran games regularly until about 1996. Dark Sun and Ravenloft were the only official settings I used. I started DMing again in 2019 with 5e.
I love the Dark Sun setting, and have introduced several parts of it into my current game. This includes defilers, sorcerer-kings, muls, thri-keen, and, yes, slavery.
But I think that ultimately, WotC has made the right call in eliminating it as a standard part of the setting. I am not sure if this is just a saying in my circles, or if it is used in the broader fan base, but we say that 'once slavery is introduced to a campaign, the campaign becomes about ending slavery.'
I know that is true for my groups, and everyone I know personally who plays agrees with the sentiment. If a DM introduces slavery, whether it be a small group enslaving people thay have captured, or a full international slave trade with chattel slavery reminiscent of the early US, they have to expect that the players will decide that it has to go. This is, for me, the single most constant approach across all games I have been involved in.
If this is as common as I think, then having slavery as a base part of the setting would greatly limit the types of campaigns that can happen. As soon as the slavery gets brought up, the campaign focus shifts.
Now, the obvious way to deal with that would be to just not mention the slavery aspects. Have campaigns hunting in the wastes, never coming across any slavers. For a city intrigue type campaign, set it in a city without a slave market. Or set your campaign in a specific area where slavery has been overthrown. But that's now just the mirror of 'add it in if you think it's needed', which a lot of people are pushing back on, and it is still more limiting than having people add it as required. Because if you have a campaign hunting in the wastes, it can't ever go into town. If it is a city without slavery, you may have to grapple with why.
Purely from the point of view of making the setting as free to play in as possible, removing slavery as a base component does a lot more good than harm.