I always assumed the died. In terms of story, Bannon may have been a leader, but he was only important for that one segment where you couldn't let him die. Arvis was more underground. They basically chilled at Narshe and eventually at Vector once the Empire claimed they were done going on the war path. However, when the party escaped, only playable characters came with. I can't remember if Arvis and Bnnon can be found after that, but maybe they were killed by the Empire?

Otherwise they were straight up killed when the world slid into ruin. Since they aren't around at all I say they are dead.

I mean, the reality is that the minor characters served their story purpose, so they were subtly written out of the narrative since the focus of the game changed from a political conflict, to a supernatural conflict, then to a worldwide conflict/rebuild/resolution.