I’m pretty sure that fade outs were the norm from the 1950s until the 1980s. For example, Music, Music, Music from 1950 by Teresa Brewer:
I think what ended it was that radio stations wanted to keep things moving, and didn’t want any “dead air”. So today, instead of keeping the fade outs, I’ve noticed that most oldies stations truncate the songs so that they no longer have the full fade out, and jump directly to the next promo or song.