Right, so Endgame premiered five years ago, right? So at least half the kids who go to watch the new films won't have seen it - they don't watch 'old movies'. Further, most of the other half, thanks to modern cultures' cultivation of the 'goldfish memory' might have seen it but won't remember much if anything about it.

Finally, according to canon, Doom can't take the mask off - it's welded to his face. Even if it's removable, the face beneath is so scarred as to be unrecognisable.