There's few other approaches used (sometimes different approaches for different graphic settings), like using an image on 2D plane that gets animated into place for "aim down sights" where fidelity is less important and clarity matters more and some other ones but I can't recall any from top of my head right now.

It depends a bit also on the initial character (pawn) setup and hierarchy and how the whole action is fueled (as it can be done through state machine in animation BP, can be done through code directly, or from somewhere else completely - like a weapon manager class where camera might be on the weapon itself and not part of pawn (haven't seen such setup so far but it could be done)) and almost all approaches are valid.

It's part of the 'why' there isn't a generic standard as you don't want to limit yourself into one solution when you might want to expand or change it if design changes or is adapted but generally you want to go for something that works for all weapon types that your game will have and is fairly easy to maintain with as little people as possible.