Yeah, once something is established in the
shared fiction (eg: we’ve all discussed it around the table) it’s there. Signed, sealed, delivered. Even metacurrency heavy games like Fabula Ultima tend to hardline established facts from player changing.
Ideas the GM has? They’re just ideas, questions, possibilities until you speak them forth and other people agree that it is so. Some systems encourage leaving lots of blanks or just “hmm, maybe …” so that as stuff happens you don’t feel like players are “denying your prep” or whatever when they come up with an interesting idea. Some have guidance on what it means to prep and how close you should hew to what’s written (as
@pemerton noted AW has some really interesting advice on how to get there for story now play).
And of course in heavy “puzzle” play like say Delta Green / CoC, prepped truth has to be super in stone for that game to work. The question here is “does D&D
require the same thing” and I think the answer is pretty clearly, apart from a well crafted and advertised puzzle type situation, no. Nothing is stopping you from playing heavy “prepped truth” but nothing is stopping you from following this rule 0 idea and doing table consensus / player fed world building.