In that case, any good GM will throw out their precious plot out of the window. It's like GMing 101: things that weren't shown on-screen exist in Schrödinger state.
IMO a good DM doesn't have any plot, never mind a precious one, in the first place.
But that doesn't mean that every game has a map full of quantum ogres until the player characters lay eyes on them and collapse the wave-function. The sandbox play-style is only meaningful if the sandbox is already pre-filled with toys such that you
aren't playing on Schrödinger's hex-map.
@Oofta
Finally, when someone else DMs, I treat them exactly they way I expect to be treated when I am DM. What they say goes. I can question it, argue it, but once the decision is final I either accept it or also have the right to leave the game.
And I don't think I would want to play in any other type of game, either.
This is pretty much my perspective. When I'm the DM, it's my turn to be in charge of what setting elements, games rules, and playable character options get used. When somebody else DMs, then it's their turn—and I don't want to play in a campaign where the DM
doesn't have a firm hand on the tiller.
Personally, I find most issues with DM authority are not so much as about refereeing the action in the game, but in the options available for the game. Especially those that don't allow for discussion, but are simply commandments handed down "from on high".
"There's no dragonborn in my game"
"I don't allow content from supplement X"
"I don't allow option X in my game; it's broken"
Well it should surprise nobody that I come at this from a very "First Edition-y," Gygaxian perspective, but—yeah, if I'm preparing the campaign, that means that I'm building a world and deciding on a set of mechanics to represent that world, and it's going to be bespoke to that campaign. Since I prefer to do all the world-building
and the game-design in order to come up with a reasonably fleshed-out milieu before I ever present the campaign idea to any potential players, they don't have much opportunity to exert any contravening "authority" over the meta-game.
But then, I refuse to play editions that have lots of player-facing rules and character-building options, so there's also a built-in expectation that the players will be selecting from a small, curated menu of playable character types.
In-game, when I'm DMing I have the final world on what happens because it's
my world and
my rules, but I also want common sense and accurate knowledge to trump those rules wherever appropriate. If a player has expertise I lack or just plain presents a good argument, I'll defer to it. But it's still my decision, and I expect the same of any DM that I play with.