I'm really unsure as to what exactly you're trying to say other than don't go into certain places in your stories. Certainly when you're doing a one-off with a group of strangers you don't go to places where things like character deaths, genocides, economic strife, ect would be used but that's more because of a time constraint. You have maybe 6 hours max with these people and lots of that is going to be slowed down by things like combat and side distractions. Those things being used as story points can't function in such a small amount of time and the story told suffers because it can't have its payoff.

I don't understand why anyone would be against something like an emotional death in a story long enough for it to be effective. It's the only way to be respectful to death in a story. If there is a death and it's someone of importance would you rather their death be unemotional? Would you rather they never die? If you've asked for there to be no deaths then you know there will be no deaths. If you know no one will ever die then anytime they're in a life threatening situation you don't have to worry and all the tension is gone. Balancing the possibility of death is a big part of why designing combat encounters is so difficult.

I think the X card is a bad idea because it's like codifying "don't be a dick" into the law. If you want to have fun and play the game you'll be accommodating. It's an insult to any decent DM or player to suggest that someone needs to have one. It pulls me out of my character to think that maybe someone could just say they don't want something and poof things are different. Everyone is making a story together, sure, but no one can be entirely in control. Not even the DM should have the power to just zap something out of existence. They have to own mistakes and bad die rolls just like the players do.

If there is a situation that is disturbing, uncomfortable, or unfun for a player then there is probably some point to it. Maybe there's a villain being set up, every good story needs an antagonist. Maybe it's a consequence of the players' actions, over throwing a leader causes lots of problems. Maybe it doesn't even involve the players but is to set motivations for an NPC, vengeance (something that would require a death to be emotional to atleast on person) is a way to give a goal. Stories that are free to explore things that aren't the normal standard subjects are the stories that are the most interesting and impactful ones. That freedom is what sets table-top apart from every other form of gaming. Allowing someone to stop that from happening is asking for worse campaigns.

If you want to use it and everyone else you're playing with has agreed to use it then go right ahead. To try and make it a rule is where the problem lies. Let anyone make any story they want because trying to stop it is impossible. If you'd rather not participate in something specific then just don't participate. Don't stop everyone else who is alright with it from going down that path.