Aha! That’s in monster_attacks.json, which is the JSON implementation of monster special attacks. Okay, now I get it. You’re right, it’s not obsolete, just underused.
attack_type can be one of leap, melee, bite, gun (as far as I can tell). special attacks can also reference one of the hardcoded attacks listed in src/monstergenerator.cpp starting around line 340.
basic attacks can just a melee skill and damage dice number and size. You only need to implement a monster_attack for attacks that do something other than bash or cut damage.