I just wish I could believe they genuinely tried. Though i'm still a bit steamed about how they tried to offhandedly murder Athas as a oh-so-funny Easter egg in Spelljammer, so I'm not terribly inclined to be charitable to WotC on the topic, I have to admit.
I could think of a number of possible ways to address the issue.
First is the Dragonlance strategy. How did the SotDQ writers address the problematic issues around the Cataclysm, or the gully dwarves? How did the Spelljammer writers deal with the problematic issues around the Unhuman Wars, or the slave-centric culture of the neogi? They ... just didn't. They made the product an adventure, and the problematic elements to the setting simply weren't present due to the location and plot they chose for the adventure. You could easily take the same strategy in Athas. Have the campaign start in post-Kalak Tyr where slavery has been abolished, and have the campaign bad guys be Dregoth in Guistenal, or ancient dwarven undead in the ruins of old Kemalok, or psurlon cults, or evil defiler nobles trying to take over where Kalak left off, or some monstrous leftover war-engine of the Cleansing Wars that has woken up again now Kalak isn't around to keep it locked away, or something similar. By restricting the campaign to these largely-non-slavery topics and areas (which are also, conveniently, largely areas where PC templars are not suited either), you could avoid the issue almost completely if you felt so inclined - and that would certainly fit the strategy of deliberate and not-un-cowardly avoidance that WotC has embraced on such matters so far.
Second is a complete retcon, a bit like the retcon they pulled on female Solamnic knights. There's no widespread slavery in Athas, there's never been widespread slavery in Athas. Instead of slaves, the gladiatorial arenas are filled with a small number of professionals or volunteers, a large number of criminals condemned to execution, and a few desperate individuals who seek a big payday for something risky, or who have sought to enter the city and by the sorcerer-kings laws must first perform a service to the city, and some jerk templar has assigned them to the arena for this purpose (plus, this makes it easier to get PCs into arena fights without having to enslave them first!). The Dragon's Tithe is routinely filled with condemned criminals and captured prisoners of war (which gives us a reason for city-states to wage war on one another). The uprising that killed Kalak was sparked by him trying to institute slavery, to help speed the construction of his ziggurat. It'd annoy a lot of people, but you could do it with surprising ease, and it'd be pretty simple to put slavery back in for your home table if you felt so inclined.
Thirdly is a serious evaluation of how slavery fits in the setting, how it operates, and the various reactions to it, and a rewrite of the setting accordingly. Have a major movement or society seeking to abolish slavery (and make it orthogonal to the Veiled Alliance, there should be escaped slaves who hate slavery but who defile, and also comfortable slave-owning nobles who are secretly Veiled Alliance members). Examine how slavery fits in the different cultures of the city-states. Gulg, for instance, may well have aboished it completely. The Oba knows her city is small and has limited space to expand in population without exhausting its resources, and she thinks it'd be foolish to bring into the city a class of people who hate the society they live in and will enthusiastically collaborate with anyone if it means they could possibly escape. Hamanu probably allows slavery (permanent or time-limited) as a judicial punishment rather than a chattel-based economic slave trade, but his law code would protect slaves as well as free on the grounds that Uriks people are its best resource and they should not be wasted unless he so chooses. Dregoth probably doesn't have slavery because he doesn't need it, all his dray are pretty much born into his cult in the first place. I've been watching a youtube series recently where a couple of Arabic gamers do a read of Al-Qadim, and they compliment how the wearing of the veil is discussed in the setting - by demonstrating how it has different forms in different places and fits differently into different cultures - that's the idea I'm trying to get at here.
There's ways to do it. I just think WotC in its current form is too risk-averse to seriously try any of them.
