As a supposed direct sequel to Decent into Avernus, they aren't taking the several endings of the module into account.
If it's any help here, there is a "standard" canon guideline for the events of game modules and how they affect the realms - and that is "The status quo remains". What this effectively means is that the presumed canon of game modules is that the great evils are thwarted in some manner, prevented from attaining the ultimate end of their plans, but they are also not permanently destroyed, and major figures survive, unless they die as part of the module's core plot. So, the evil lich fails to cover the world in her ultimate necrotic shroud, but her phylactery is also not destroyed. Laeral Silverhand is not killed in the assault on her home, but neither is Manshoon (at least, not all of him), etc...
In terms of what this means for the Avernus adventure, the formal post-adventure canon is presumed to be:
- Elturel was dragged into the hells, but it was also returned, thanks to the efforts of heroes. - No lords of the hells were slain permanently. - Important NPCs mentioned in the adventure all survived, unless their deaths were ordained by the module itself.
and, unfortunately,
- Zariel was neither slain, nor redeemed, and remains in her position as the ruler of Avernus (sorry Lulu).
Individual players and tables will have their own evolving worlds where their adventures changed the course of history and the face of the realms, but in terms of the writing of future official adventures, the standard canon is always that the actions of the heroes in previous modules avert disaster, but status quo of other elements remains.
