I've come to realize (I think) that this actually does have a lot to do with waterproofness ratings -- a legibility trap.

I notice that Fairphone excludes headphones from their latest devices, and attributes it to the necessary of doing so in order to get an "IP55" rating.

I'm not sure if that ultimately makes sense (and suspect that it... doesn't), but the legibility trap of that ratings system might actually be part of the cause of the current market absence of a feature so many people still talk about after years of its unavailability.