What Prevents load voltage from Flowing into Vsupply in this circuit?
In a steady state there will be no current from the high voltage supply through the gate side since there is only one DC connection between the two. The gate itself being capacitive.

However: There is an AC path though the gate capacitance that will introduce a voltage on the gate when the high voltage supply is initially attached or powered on. That coupling will briefly turn on the MOSFET and apply power to the load. How long that pulse is will depend on how active the opto-coupler is at that point.



If the load is fast enough to react to that pulse, the effect can be problematic.

Inclusion of a gate protection zener as shown below is also prudent.







simulate this circuit &ndash; Schematic created using CircuitLab

As Tony mentions in his answer, any AC noise on the high voltage supply line will also couple though the MOSFET to the gate side to some extent.