I especially don't think characters should have any way of getting themselves killed instantly by another character, which is controlled from the victim's side. That seems ripe for abuse.
To say nothing of characters wanting to die as a default reaction to everything.
I think this sort of mechanic makes more sense in a game where players are collaborating OOC to make scenes play out in a specific way, but makes less sense when players are competing with one another and are not really supposed to know one another's true OOC intentions.
I can think of a lot more past situations where apparently vulnerable characters turned out to be anything, and there is at least one famous instance of a character escaping a planned execution when they were notionally 'going along with it' thanks to clutch combat rolls at the last second.
I think both signalling and non-signalling versions of a mechanic like this have different negatives. I get the thinking here but I personally think the situations in which players might use it to dubious purposes outnumber those where it salvages pre-planned guillotine scenes.
And if your problem is with instant death and eggshell skulls, well... Let's not have it be instant death. Let's have it be that you make no attempt to dodge, that you make no attempt to fight back or parry, that attacks have a greater chance to hit the face, head and chest, and that damage is multiplied, maybe by three.
Hell, that would even work as a power move, allowing someone a free hit that does triple damage. A weak immy with his fists wouldn't be able to kill a hardened badass even with triple damage, whereas a gun user would fare much better at doing a proper execution (and if you're using guns then you should be okay with killing).
I feel like @holdback 60, sit, and stop attacking allows for a very submissive roleplaying posture without any potential for shenanigans, but it may be that the need for cooperative executions is far greater than I've experienced.