My impression is that Imaginary Stories were rare in the post-Weisinger period. Anyone know offhand what was the last Imaginary Story published before "Whatever Happened to the Man of Tomorrow"?
Would something like SUPERMAN ANNUAL #11, “For the Man Who Has Everything.” (1985) count where thanks to the Black Mercy Superman lives a life where Kryton did not explode? It follows a plot device used in Imaginary Stories like "Superman's Other Life!" in (Superman V1 #132, 1959) or World's Finest #263 (1980) where the entire series of Super Son stories were revealed to be an elaborate computer program?
I think the reason you saw the demise of 'Imaginary Stories' by the 1970s was that you had alternate earths as a way to create a story without messing us 'regular' continuity. Want a Earth where Batman is the only hero? Allow the Batman of Earht-1 to change the history of another Earth ("To Kill a Legend"). Want to tell the story where Superman is actually the second and last of a line of matrix like beings? Have him pulled to the future of another earth as happend in "Superman you are Dead, Dead, Dead!" Want Superman to finally marry Lois Lane? Have it happen on Earth-2 as per "Superman takes a Wife."
Even better if you did do something that messed with continuity some editor could after the fact create another alternate Earth so future writers did not have to deal with the problem. This is how Earth-2A (called Earth-E here) and Earth-B came into existance. With all these earths to play with you didn't need the out of an 'Imaginary Stories'