GURPS Martial Arts 43 mentions this as a limitation for Damage Resistance
Page 47 discusses taking it for Regeneration
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2017 http://forums.sjgames.com/showpost.php?p=2106337&postcount=2
I'd call "Accessibility, Humanoid* unarmed attacks" -40%, personally, as they're uncommon due to being relatively less effective. So one subset of that (just kicks, punches, grabs, or other) would be another -20%. So -60% total.
- To differentiate from being attacked by animals, which is a very different kettle of fish.