"That's a rather heroic claim. Fortunately, "from what I've been told" doesn't constitute evidence. For me to accept your claim, I would need something more convincing than that."
I recorded with an engineer that recorded many of the stars from the 60s and 70s and if I remember correctly he said everyone doubled their vocals. It was a common practice once multi track recording came into play. If the lead vocalist isn't doubling there's background singers doing it. Rock records usually always have doubled vocals to make the vocals fuller. That's what doubling is used for (or at least that's how it was explained to me). Once I knew that, I could hear it. Because you rarely hear a doubled vocal that's perfect.
"Bruce Springsteen Nebraska is a very obvious'
Superstar Springsteen could cough for an hour on tape and still have a gold album. Did you ever hear Two Virgins?
"(Smiths, Beck, Violent Femmes, etc)"
Hate all these bands, so I don't think I've ever heard a full song by any. So, maybe they don't. But, bands that don't are far outnumbered by ones that do.
While I might be in the minority. Someone name at least 5 TOP TEN HITS from the 60s - 80s that only contains one single vocal. Someone mentioned Bohemian Rhapsody. How many vocal parts are on that? It doesn't need doubling, it's full by the sheer number of tracks.