The DM saying causing an attempt at an impossible task to fail isn't removing agency. The players still have the same agency that they had prior to saying not. It was just appropriately limited by game rules, just like in any RPG.
<snipP
Going by the D&D rules, though, saying no to an impossible action doesn't remove/negate any player agency. That agency was taken by RAW, not the DM.
In D&D 5E (which is the game currently at the top of my brain, so it's easiest for me to reference) the fact the DM decides if something is certain or in doubt, and the difficulty if it's in doubt, is called out in the Player's Handbook, so it seems to me like a player-facing thing. It is of course, possible that I'm misunderstanding you--or that I'm one of those GMs who doesn't allow player agency at all.
These posts seeem confused about
@chaochou's point. He is not making an assertion about
what the rules of 5e D&D permit or require. He is making an assertion about
whether a particular decision-making procedure permits players to exercise agency.
It does not rebut his claim to show that one popular RPG endorses or promotes that decision-making procedure.
Is there a game where the players have the ability to change everything and anything that they like, or are there rules and limitations on what and how they can change them? Because I haven't seen a game you've mentioned where the players can do anything they like. At the very least they aren't allowed to undo established things.
In the post of mine that you quotd, I distinguished two things:
(i) Establishing the costraints of genre and fictional positioning;
(ii) Applying the action resolution mechanics to find out what happens when an action is declared.
The first - which seems to be what you are referring to when you talk about "changing everything" and "established things" -
does engage player agency. Because it is (or certainly can be) a matter of negotiation and table consensus.
The second does (or certainly
can) engage player agency because the action resolution mechanics tell us whether the player's vision or the GM's vision of what comes next prevails.
Agency is limited in pretty much all RPGs.
Which RPGs do you have in mind?
In a social activity - include collective generation of a fiction as takes place in RPGing - it will be rare for any one person to have everything play out as they envisage it. But that isn't what
@chaochou is talking about. He referred to
GM decides - that is, to a situation in which one person routinely gets to have things play out as they envisage it. That clearly involves a burden on the agency of other participants.