My own theory is that postmodernism is much much older. Even some of the Mannerist art from the late Renaissance has that sort of conceptual dissonance to me.[0]

The four big pillars are romanticism, which is focused on the past; realism is concerned with the present, modernism is focused on the future. And then postmodernism exists as some sort of combination of the three, combining elements of each in surprising ways -- sometimes a synthesis, other times a rejection.

Of course all my university education is in physical science so this is probably stupid and wrong.

[0]: Seriously, look at this one: https://en.wikipedia.org/wiki/The_Librarian_(painting)