If Bane was a greater power still why on Toril would he relinquish that to be a suped up mortal? Unless Ao forced him and the others into that status. I sort of stopped paying attention after 3.5 and it seems like WOTC have gotten lazy and sloppy in their narrative.
I think the idea is that the dead three refused to stop meddling with affairs on faerun after Ao decreed that the gods are supposed to withdraw and no longer directly intervene in the affairs of mortals. Thus Ao made them mortal.
I suspect the scant info on this available is because it's going to developed in this game itself. I suspect the appearance of the dead three in Descent into Avernus was purely to help set the stage for BG3.
