Two extreme examples: a player says that their character would like a new mountain range to appear, without reference to any specific rules, and the dm says, "no, that's not possible." Is that indicative of a denial of player agency? Or, a player says their character would like to go talk to the blacksmith, and the dm says, "rocks fall, you die" for seemingly no reason. Is that outside the purview of the dm, who is in control of the world?
I think that these examples are underspecified.
The main way that a player contributes to the shared fiction, in D&D play, is by way of
PC building and by
playing their PC by declaring actions.
So in your mountain range example, what is going on? If we're talking PC build, and the player says
I want to play a person from the mountains, then that is a discussion about setting and backstory. I think that the default GM reply should be OK, but obviously there might be reasons to say no or to seek some sort of compromise. (Eg everyone's agreed to play a campaign of urban scoundrels living in their home city which is also an ocean port.)
If we're talking action declaration, then what is the declared action?
I look for a mountain,
I travel to the mountains, I look around - do I see any mountains?, I say to the NPC "Like you, I am from the mountains" - these are all different action declarations, and for each there are various ways they might be resolved. There are ways of giving players agency here which don't extend to mere fiat player authorship. There are also ways of preserving GM authority over backstory and setting - as is typical in a "sandbox" game.
With the blacksmith example, the player declares an action:
I go to visit the blacksmith. What is the fictional positioning? What is established already? If the fictional positioning seems propitious - eg the PC is in a town of reasonable size, with plenty of money to hand and under no particular time pressure - and yet the GM is inclined to say anything other than
OK then, why is the GM so inclined? The player wants a scene of their PC meeting the blacksmith, but the GM is refusing to frame it - why? To what end? Based on what sort of assertion of authority?
Here are two possibilities: (1) The GM is just asserting authority over backstory (and letting that bleed through to situation) because of some inner sense of the "integrity" of their creative vision, or because they happen not to have thought of it first in preparing their notes. I personally think that is bad GMing. (2) The GM's response is that
there's no blacksmith - and the town is in consternation as a result!, and this is the introduction to some sort of mystery or adventure. That may be good GMing or not depending on whether anyone is interested in that adventure and similar considerations. But as far as the exercise of authority is concerned, (2) is completely different from (1). And progress can't be made, in discussions of GM authority, until we start to drill down to these sorts of differences.