> A Canticle for Liebowitz for example mostly feels like some kind of fantasy but for the fact that the reader knows it's set in a post-apocalyptic nuclear wasteland.

You might compare The Sword of Shannara, which is a character-for-character, scene-for-scene copy of Lord of the Rings, but which is technically set in the postapocalyptic future (it's easy to read the book without noticing this) rather than the legendary past.