If you try to bash the door down and fail the roll, your declaration fails in the fiction just the same as if the DM says no. If the roll succeeds, it succeeds in the fiction just the same as if the DM says yes.
I don't see how relying on a random roll grants you agency with your declaration. Either the die roll says yes or no, or the DM says yes or no. Either way you are dependent on something outside of your control(barring the ability to re-roll or something which gives some limited control). If one method that results in failure equals no agency, then the other equals no agency as well.
Let me start off by saying that what I think constitutes agency is more involved than the examples I was giving in reply to the question about the Teleport spell. I saw that question as an attempt to find some basis....some foundation, and so I answered it. But I think there are a lot more elements that generally need to be considered.
I don't think that I'd say that a dice roll and the GM deciding are equally "outside of your control" as you suggest. With a dice roll, generally speaking, as a player I'm going to have some idea of my chances of success....certainly I'll know how good my character may be at a given task. For example, to kick in the door, my character has a high Strength and so it's reasonable he can do it. The GM will likely (hopefully) also share some details on the door that will help inform my decision. Ideally, he'd state what the DC of the check would be. And so on. So I can then as a player calculate my odds and the risk of failure, and can then proceed with the attempt or not. I'm making an informed decision.
If the GM decides, then I likely have a less clear idea of my chances. That may not be the case....I may still have a good idea of my chances because the GM has told me the door is made of flimsy wood and does no appear to be barred from the other side, and he's hopefully going to factor in my high Strength score and so on. If so, great. This is much more simple with certain actions than others, and the door kicking example is an easy one for sure.
Imagine a more complex action declaration, maybe of the sort offered in the OP.....a PC insulting a NPC. Now, we don't know exactly what the intent of the insult was in the OP, but let's imagine it had a purpose. Let's treat it as an intimidate check trying to convince the baron to negotiate with the PCs by letting them know if he doesn't, they're not fond of him and may act against him.
In such an example under the GM decides method, I may have no real sense for chance of success or consequence of failure or anything else. It's a much less informed decision. The GM can choose to narrate a result that I may not have thought was on the table.
No he can't. Just like he can't approve every action. Either of those things would be a blatant violation of the social contract and the game rules. The DM like the players, has to act in good faith with his rulings.
Earlier in the thread, many people said that the DM controls when the mechanics get invoked. The DM decides if a declared action is outright successful, or if a roll is needed to determine success, or if the action is outright impossible. This gives the DM all authority on when the dice are rolled. Yes, we would expect and hope that the DM would use this authority in good faith and with principles guiding him in some way. But these factors are going to vary much more than game mechanics, no?
If you're playing with the same group you have been for some time, you may have a very strong sense of how your DM may judge these things. And that's great. I'm lucky enough for that to be the case for me and my group. But even still, at times conflict still comes up. No one is going to be 100% consistent. And no two people are going to agree on what 100% consistent may mean.
Now, if you are playing with a group that is new to you.....you have far less past experience to guide you here, so it becomes even less clear. How can you say what your expectations should be under this system? It's a much bigger gray area.
So, although yes, shooting down every idea is bad faith play and I doubt such a DM would keep a game together, but it doesn't require bad faith play for there to be less player agency under such a system. The DM could be doing everything that he thinks is right, and another DM could also be doing everything he thinks is right, and you can have two different ways it plays out.
Compared to a game that adopts the previously mentioned principle of "say yes, or roll the dice", this method is much less clear.
I look at it as...
1) The players declare how they want to try and change the fiction.
2) For the vast majority of declarations, both success and failure change and shape the fiction, so virtually every declaration, regardless of auto success, auto failure, or die roll to determine, succeeds in changing the game world.
3) Since pretty much every declaration will change the fiction somehow, the ability to make declarations gives them agency. They have full control over how their character will shape the fiction through both successes and failures.
I don't really agree with your number 2, and therefore your conclusion 3. Doesn't that render the idea of player agency moot? When is there not player agency in such a broad application? Also, the bit about players "having full control over how their character will shape the fiction through both successes and failure"; I don't think this exists in D&D, in many cases. The player doesn't decide that when he fails to kick down the door, he breaks his foot, or he makes such a ruckus that it attracts a wandering monster, or any other result of the failure. The DM decides that.
Are you saying that player agency is only taken away if control of a character is taken from them? If so, does this apply to such in game effects as Charm and Dominate Person? Or only when a DM just decides out of the blue "okay, your PC is a NPC now.....too bad, make a new guy"? I suppose this may happen in very rare instances where a PC's alignment shifts to evil in a campaign where the group has agreed for no evil characters. I mean, I'd expect that in such an instance, it would be very obvious to the player that this was the result of his character's actions, and so he'd be choosing to allow this.....but if there are other examples, I'd be interested to hear. I can't really think of any.
I don't see how this broad of a read of agency is meaningful. Otherwise, it sounds like every RPG has an equal amount of player agency....and I don't think that's the case at all.