I think this discussion kind of encompasses multiple topics, some of which aren’t exactly saying no as much as reaching a consensus.
For example, if you, as a GM, says, “I don’t want to use ancestry X or class Y because [reason],” and the player says “I think Y actually fits because [reason],” that’s a discussion, and while the player desires are important, the GM is also a player at the table that needs to enjoy the game, and is an important voice. The GM needs to present their ideas, but should probably be open to a discussion of their reasoning.
Discussing player actions and consequences is also maybe not really about “no,” but about making sure that every one is clear about what is about to happen. Sometimes this discussion isn’t just about “your character wouldn’t do that,” because, ultimately, that is the player’s choice, but what the GM and other players have a say in is “do we want the campaign to go this direction.” If someone is about to kill someone, and the whole team would become criminals on the run, that’s something to have a meta-discussion about, which isn’t even about “would you do this,” and is more about “are we okay with one player changing the paradigm because how they want their character to act.”
Related, but different is a safety discussion.
“I’m going to torture this captive.”
“I really don’t want to be at a table where this is going to be an option. I’m uncomfortable with this.”
While it’s a no, its a no about someone saying that they may not be emotionally or mentally okay with addressing a topic in a game, which is different than just discussions about genre emulation or even storytelling trajectories. Like the GM that says what rules will and won’t get used, this is something you can at least start to address in a session zero by establishing lines and veils, but without the negotiations you can have for rules. If that person isn’t comfortable with a topic, and you want them at the table, you can’t include that content.
Then there is the very simple yes and no that GMs just have to adjudicate.
“If I throw a dagger at that barrel of unstable alchemical materials, can I get it to blow up?”
“No, you still need to apply fire or some kind of catalyst.”
“Can I throw my gun to another player, in such a way that they get to fire it because I hit their finger, but I’m rolling my throwing skill to attack but I get their bonus to gun damage?”
" . . . no"