@FrogReaver
So here is the scenario
at the table: the GM presents, in whatever fashion, the opportunity for the player to have their PC travel to an unknown demiplane with unknown characteristics (eg there's a Well of Many Worlds or something like that). The player declares that their PC takes up the opportunity (eg steps through the portal, dives into the Well, or whatever else). Now the GM tells the player - at some appropriate juncture, such as when the player has their PC look for eggs - that there are none, as it's a lifeless demiplane.
To me that is obviously low player agency RPGing. The player is just prompting the GM to reveal the GM's conception of the fiction. The player is not establishing that fiction.