> there aren't many things more stressful than getting punched in the face

I can definitely see this happening in dangerous/adrenaline sports like climbing. Normal things and fears just don't bother me anymore, when you are continuously facing your fear of grave injury and/or death when abyss stares at you and you hold just on your finger tips. There is rope but that doesn't do much for the fear, and you can still get injured if fall is nasty. And its not just exposure to fear, but you need to semi-continuously keep overcoming it during entire session, repeat that 1..X times per week, and after few years you become somebody else in this regard (and few others).

Now-famous Alex Honnold said that his fear receptors in brain basically just don't trigger anymore - they did some MRI scans of his brain. But the thing is, there was nothing special apart from that, and he himself attributes this to 2 decades+ of daily exposure to increasingly dangerous situations (not just famous crazy free solos but a lot of wild scrambling which can, and often does turn into serious exposed solo climbing without a chance to retreat).

Brain is a muscle, and fear is one dimension of how to expose and train it. Too much too quickly and it will overwhelm anybody. Bud to it gradually and in dosed manner and things will happen.

So glad you brought up danger/adrenaline addicts - in my head there is a stereotype of the personality type, they exude a very chill energy and I've always wondered if their lifestyle meant they had become numb to normal life.

The anecdote about Alex Honnold not having fear receptors sounds on the mark