The built-in kill switch feature has been implemented and tested on esk8, remains to be tested on efoils:
App Settings >> General. You can choose PPM low, PPM high, ADC 2 low, ADC2 high.
How to enable kill switch support on Firmware 5.03
https://vesc-project.com/node/3426
Lanyard kill switch:
https://vesc-project.com/node/3356