You're I'm a group. You order pizza. Seafood pizza player vetoed all the pizza options that aren't a variety of seafood pizza because that's what they like.
Ir if a variety is ordered they get a seafood pizza and eat everyone else's pizza leaving their pizza at the end for themselves.
Yes I've seen this happen not theoret.
Okay, so that’s just someone who’s being a jerk, but it really doesn’t have any relevance to what you’re talking about. And to your point, a consent form is just a way of indicating what a game is going to be about, though I’ve never heard of a player giving one to a DM before and I’m not really sure how that would work: the player doesn’t control the content of the game.
Now if the “consent form” as you call it is a list of things that they’d rather not deal with in a game for whatever reason, I understand that. Not every DM does a session zero, not every DM communicates what kind of content is going to be in their game, and to be fair, sometimes it’s difficult to cover all the bases.
I had a game with someone who has severe arachnophobia and was visibly squeamish about fighting giant spiders. You always think it’s not gonna come up, and you think it’s something that will just blow over and they can cope, but no. This was really bothering them and they weren’t angry, they were deeply embarrassed and upset that they were being a downer for the group. They felt bad. I felt bad. I switched tactics quickly and had the spiders chased away by the “real” monster that the party had to face - a quick pivot. Everything simmered down and we got on with the game. So in retrospect, a player coming to me and saying “Hey, just so you know, I’ve got a really bad fear of spiders and just wanted to let you know in case it’s a part of your game you have planned” would actually have been a great thing. But they didn’t think of it. I didn’t think of it. Don’t dismiss the idea of players coming pre-armed with a list of things that are red flags for them.