Between 0.16 and 0.20 set_user_hitzones reverted to how it was (and probably still is) from OLO's AMX Mod. I created a small plugin to test this. It's getting popular. :-)
Set HS only mode

"amx_hitzones 3"
Display help

"amx_hitzoneshelp"
The difference is you now can specify bodyparts "inter-individually", ie you could set it up to allow player A to hit everyone anywhere but only hit player B in head, and player B could only hit people in their legs.
This test plugin just sets the same hitzones on everyone though. The generic hitzone should always (?) be used, dunno really what it is used for. Head is 2 and generic is 1, so for a HS only mode you use 3.