I don't think is a recent phenomenon though. Good luck distinguishing by voice alone the boy bands of the 90s, or the hair bands of the 80s, or the disco bands

I don't think is a recent phenomenon though. Good luck distinguishing by voice alone the boy bands of the 90s, or the hair bands of the 80s, or the disco bands of the 70s, or the British invasion bands of the 60s either. And going back much further, can you really say you can distinguish the operatic and choral singers from each other either? I think it's always been the case that music has a particular sound that's in, and everyone is singing to that sound. There was no one singing in the grunge or mumble rap style a hundred years ago. It would have been unique and would've easily separated itself from the pack, but it simply wasn't even a thing that anyone was doing yet.