A lot of people keep mentioning Dark Sun, but evidence suggests Athas is not only not on the slate for 2021, but it’s likely not even one of the 3 settings being worked on.
Look no further than Tasha’s for the tells. It’s filled with references to other settings. The subclasses hold multiple references to different planes. The legendary magic items have origins from Greyhawk and Ravenloft. Azalin the Lich (Ravenloft) and Guildmaster Rhys (Planescape) get art depictions.
Dream of the Blue Vail, a spell made to allow multiversal travel, mentions settings like Greyhawk, Dragonlance, and Eberron.
But how many references does Athas get? Zero. None. Nada. Not a single mention (by name) in any subclass, magic item, spell... nothing. At best, there’s a single art piece that looks like it may be an Athas reference, but even that piece doesn’t mention Dark Sun or Athas by name.
Maybe its the struggle WotC is having with offering satisfying Psionic options. Maybe it’s the inherent difficulty with tackling Athas in the current climate (slavery, among many other things, don’t play well in 2021). Whatever the case, WotC has seemingly scrubbed their products clean of Athas references and don’t seem like they wanna tackle the setting anytime soon.