The only album that comes to mind is Exile on Main Street, but I'm not particularly into back stories. Were a lot of people doing this in the 70s?

I'm D'Angelo is as good as it gets. Voodoo and Black Messiah are two of my absolute favorite albums, and they're the only two albums he's put out in the last 20 years. Other than it being a long process, I don't know a lot about the Black Messiah recording process, but Voodoo was multiple years of jamming with super-talented musicians then condensing that material into an album. They'd listen to classics like There's a Riot Going On then use that inspiration to guide the jam sessions. I think this approach is pretty similar to what you're referring to. On the other hand, look at how quickly acts like The Beatles, Bob Dylan, and the Stones put out music in the 60s. The Beatles released two albums in '67 then a double-album in '68; Dylan released two albums in '65 then a double-album in '66 - then recorded all the Basement Tapes material in '67 and released another album that same year.