Has that ever been a thing? Quoting (and trimming) the message you are
replying to has been normal since at least the late 1980's (when I
started to use E-Mail).
Peter,
My memory of email spans only a slightly larger period but no doubt yours is better than mine. My main reader was emac's rmail until too much mail wasn't text. In my memory you had to turn on quoting the original (though irrc rmail had an insert-original command to allow one to do it as necessary - and at "point"). I see today that Thunderbird sets quoting on by default (per account) but it is optional. But it can also display an entire thread in the correct who-replied-to-whom. I get that often there's a particular sub-point of the original which needs to be at hand in the reply but rarely the entire message - so maybe we're back to it's a Netiquette thing (and in my case at least often a laziness thing).