Shin Kicks are a combat option mentioned on GURPS Martial Arts page 112. It is an option for Jam and Kicking and Spinning Kick.
The benefit it offers is:
- giving damage bonuses from leg armor instead of foot armor.
- damage from Aggressive Parry is to leg instead of foot
Although not explicitly allowed for Drop Kick, that would be a way to emulate the Guillotine Drop technique from Ring Dream.