Modern albums are often labeled with an “E” when they contain explicit content. Was there an analogous practice for albums released earlier in the 20th century?

No.

Sometimes album covers were censored, tho. Look for Electric Ladyland by Jimi Hendrix. Or Sticky Fingers by Rolling Stones. This one had not only blursed lyrics, but even songs' titles. Like Brown Sugar and Sister Morphine.

A famous cover that would not be accepted even now is Blind Faith, 1969. NSFW.