Juniper Bug

I was trying to think of things where this isn't true, and the only example I could come up with was Death Stranding, in which the game Death Stranding was implied to have existed before the apocalyptic event (also called the Death Stranding)

Marvel actually does have the in-universe comics accounted for; they're licensed and they've done at least one story about suing the makers for libel.

There's a short alternate-universe Superman story called Secret Identity where a kid gets Superman powers in a world where Superman comics exist and everyone knows about them. it's kinda maudlin and politically weird but it gets at the tension in the middle of what you're talking about that's fun.

And then there's the worlds where stuff DOES exist because the heroes or whatever are the ones the content is based on. :3

And then you get Human Centipede where HC doesn't exist in the first movie and what happens in the second is based on the 1st being a movie and then in 3, both are movies.

There's the trivia that because the JJ Abrams Star Trek used "Sabotage," surely that universe must also have "Intergalactic," which refers to Mr Spock and his Vulcan nerve pinch

splatoon canonically existed in the pre-apocalypse splatoon universe, and this barely matters to the lore at all

iirc in Watchmen, because superheroes are real, most comics are about pirates