Series diode in power supplies
Maybe I don't understand what you're asking, but I don't think that diode does what you think it does...

There will be capacitance at VOUT of the regulator.  Probably 100uF or more.  Then there will also often be a bunch of other capacitors at other places, all in parallel.

So when the input power is removed, all this capacitance will hold a voltage at VOUT until the load finally dissipates it.  If the load is a CPU in "sleep" or low-power mode, that capacitance may not drain very fast.  It may be so slow, you end up with a 'brown out' or a poor reset.  It's best to drain the capacitance fast as possible.

Many voltage regulators will not pass current from "VOUT" to "VIN", even if the voltage at VOUT is higher than VIN.  So to give the capacitance a path to drain fast, that reverse polarity diode is added.   Of course, for this to work there has to be some resistance from VIN to ground.  That's easy to implement if the power switch is a 1P2T.