Just as another data point, Rockwheel GT16 also has these "80% beeps", and these beeps are not controlled by Rockwheel app or Wheellog. It beeps when starting hard from standstill or going up very steep hill. These beeps have nothing to do with speed or battery level. Since Rockwheel and Gotway were one company (Gotway boss left RW and started his own company), Gotway firmware evolution started from the same code base as Rockwheel, so they probably still share the same philosophy - 80% load beeps, giving the user all the power that hardware can produce, no FW updates (no need - "it just works"), etc.
My guess is that these 80% beeps on Gotway and Rockwheel hoverwheels simply indicate 80%+ PWM level, you have less than 20% margin left.