In a twenty years old fortress. There has been three notable death from helm removal. Notable because these dwarves were fully equipped elites with legendary in many combat skills.
First one charged a whole siege alone, took out a couple of enemies before getting hit in the left arm, dropping shield, another hit in the right, dropping crossbow, got his helm grabbed and removed, then got an axe to the head. Only five enemy actions landed in that fight (Two arm hits, grab, remove helm, and an axe) with the rest got negated by shield, parry, dodge, or armor deflect.
Second one was fighting in a regular melee against goblin siege. Didn't even over extended. Wasn't even injured one bit until he got his helm grabbed, removed, and got hit in the head.
Latest one got berserk because he couldn't get a ball of thread and a piece of leather... despite the fort drowning in leather and thread... Probably not the right kind... He promptly got swarmed by ~90 militias and 29 soldiers very quickly. Most attack were blocked, dodged, or parried. Those that did land were deflected and did minimal damage. Until one soldier decided to stop hitting the berserk dwarf with weapon and start wrestling. His first move was remove the hood, another was remove the helm. Then a miner militia jam a pick into the berserk dwarf head ending the fight.
Three elites died, all because they got their helm removed. And I never lost an elite dwarf to any other kind of death in combat against regular humanoids.