Which component/s determine the maximum voltage of an ESC?
Vgs is probably not the limiting factor.  Most high voltage ESCs use NFETS on both the high and low side switches and use gate drivers.  The gate drivers generate the gate drive voltage above the positive rail to fully saturate the mosfet.  All the gate drivers regulate the max Vgs voltage to insure the FETS are fully saturated with prevent damage from excessive Vgs.

The limiting factor will be the weakest link in the circuit, typically the filter caps, the buck converter to generate the internal Vcc for the ESC or the Vds of the switch MOSFETs.  The MOSFETS have to be rated at a higher than the battery voltage because there is some inductive energy dump from the motor coils during switch-off and while braking.