What is it with fantasy stories suddenly throwing in "Oh yeah, this is ACTUALLY a post apocalypse story where the modern civilization fell apart thousands of years ago."
It's interesting if that's part of the main premise, but this is chapter 26 and we've seen 0 mention of this anywhere leading up to this.
It can be done well with zero mention a the start, but it has to be a slow burn introduction of these elements, at minimum as slow as done in "Kumo Desuga Nani Ka?" You have to introduce one or two things that make people speculate about it, and then give a couple of more breadcrumbs over the course of at least two or more arcs before doing the big reveal. Otherwise, it just feels like you're rug-pulling the audience when this stuff takes center stage/the original premise is "abandoned."
Honestly, I've seen this kind of "bait-and-switch" often enough lately that I'm starting to think a bunch of these were cases of: "I want to write x story, but y story is popular these days, so I'll make a y story at the start and turn into x story once I get people invested enough to keep reading."