In & out of fashion over the years, the fade-out has been around for decades—centuries if you include live musical performances. Not really a thing in the early decades of recorded music, only with the advent of magnetic tape recording did it became possible to fade a song out simply by pulling the faders on the mixing board.
The fade-out technique wasn't invented because musicians didn't know how to end a song. The consensus seems to be it was developed to appease radio programmers. FULL STORY