> Distortion (other than perhaps mild distortion) has never been fully accepted in roles where the multiple voices of a complex harmony part would be distorted together.

I'll fight this by agreeing with you. Saturation (a form of mild distortion) on the master is what I like to refer to as "the shit" because a nearly imperceptible amount really makes a track cohere. It takes a super light touch though, but I'd bet (without evidence) it's used on most tracks today.

Here I would posit that even our ear canals distort a bit, particularly when things get loud. It possibly helps us zero in on pitch.

> I'd bet (without evidence) it's used on most tracks today.

The Aphex Aural Exciter supposedly adds harmonics related to the signal by generating distortion in a certain way. That's been around since the 70s.