Backgrounds are some of the least backward compatible aspects of 5.5, and the main way they expressed Knights of Solamnia and Mages of High Sorcery were through backgrounds, so at the very least, I would expect them to revise some those backgrounds, if DL was on the near horizon. They've been teasing Dark Sun since before Ray Winninger left. Dark Sun is the only campaign setting mentioned in the 2024 Dungeon Master's Guide that didn't have some form of update for either 2014 or 5.5. Last August we explicitly got the Apocalyptic Subclasses that included the Preserver Druid, the Gladiator Fighter, the Defiler Sorcerer, and the Sorcerer-King patron Warlock. That's a whole lot of work to leave sitting on the table if Dark Sun isn't fairly close showing up.
Regarding setting books versus adventures, Forgotten Realms, Eberron, and Ravenloft all had setting books in addition to adventure support (or in lieu of adventure support, in the case of Eberron). But I don't know that they assume that any setting beyond some of the biggest, most popular ones are worth doign a full sourcebook for. They literally didn't do a separate planar handbook or Planescape setting book, they did the 3-Book Slipcase model to facilitate an adventure.
If we look at what they did with Dragonlance, let's look at potential problematic content in Dragonlance:
- The portrayal of the Plainsfolk and Kagonesti elves
- Gully dwarves, their origins, and what that implies in a broader sense
- Religious elements like the Disks of Mishakal that are literally parallel to LDS aspects
- The racial confict between Silvanesti and Qualinesti elves and Hill dwarves and Mountain dwarves
Making
Shadow of the Dragon Queen into an adventure means they didn't have to touch on any elements of the setting that weren't relevant to the adventure they wrote. That's exactly the kind of thing you could do if you provided just enough setting information to run the specific adventure you have written for Dark Sun. If the Champions of Rajaat literally commiting genocide isn't an thing you want to touch on, a specific adventure isn't likely to need to reference that, unless you make the adventure about that specific aspect of history.
