I agree with the picture of the click described above.
And as to how long it has been "norm", I suggest first half of the 80s. I think it started as a necessity of keeping the tempo within the very same BPM through the song (or throughout a given section), because you wanted loops or other timed effects, certainly MIDI made an impact here, to be able to run in parallel to the performance. When this was becoming popular, people in production slowly changed the notion of "keeping the time" from 'steady pulse
in relation to a song' to 'staying on the same BPM throughout'. It's a "production" decease, if you will.
Far too many see it as "keeping the tempo" or "straightening up a roving rhythm section". However, usually a roving musician will deliver a more spineless performance when presented with the need to chase a click, so as a remedy for wavering performance click usually doesn't work very well. A favorite moment for me was a Dave Weckl clinic about 20 years ago, where he demonstrated playing to a click in such a way that he made the click sound like it was chronically rushing, and then switched to where he made the click sound like chronically dragging - even though his groove and the click's tempo was constant (not talking about his trademark 'groove displacement'). So it's definitely possible to screw things up using a click.
And as far as keeping the tempo, we should remember that the human perception of steady pulse
is not absolutely straight. If a good rhythm section is able to play with just the right tempo modulations over the relevant emotional landscape of a song, you will perceive it as much more steady and 'spine-ful' than had they kept the tempo dead straight. Keeping it dead straight through a song section change may very well sound like dragging, or rushing. This is a deliberate musical ability that many good musicians knows, but unfortunately very few producers know of, or dare to trust musician to utilize.
There's this story about a well known studio drummer in sweden, who was recording, playing to the perc loop of a Linn drum machine. Early 80s. At the main chorus the drummer's groove dropped behind the loop. Happened every time. But the drummer came in saying the drum machine rushes away at the main chorus, and got both frowns and smirks and chuckles in return. But he insisted, so much that in the end they put up 2 Linn drum machines in parallel and played the song to prove him wrong, and when the main chorus came one of the Linns pulled away from the other. After that he was accredited with the ears of a rhythm god, but himself he says that's not it. He didn't use his 'tempo ear' to pick it up; it was emotional, musical, not technical.