My CanAm used to do that because the ignition was hooked up wrong. Think about it, from the perspective of the porting/carb there is no difference whether the engine is running forward or backward. The only difference is the ignition should fire before the piston hits TDC (top dead center) to make maximum power but if the timing is close to TDC or the ignition is roughly symmetric around TDC then the engine will run just fine backwards. For example if a points ignition opens 15 degrees before TDC and then closes the points 15 degrees after TDC then the timing will look exactly the same with the engine running in the other direction.

I don't think there is a diode that will prevent the bike from running in the opposite direction as the poles of the magnets in the magneto alternate. I think what you are thinking of is a shunt voltage regulator. A diode will not turn on and pass current until it sees a certain forward voltage across the diode. For a silicon diode this is ~.6 volts. For the old selenium diodes it's closer to 1V so if you stack 6 selenium diodes across the lighting/ignition/charging system any voltage over 6V will get shunted across the diode to ground effectively limiting the voltage to 6V. A cheap and effective solution and very common in older ignition systems.

Makes you wonder.

Would a simple switch be able to make a bike go backwards if needed. Maybe to back up out of water/ mud or some other tricky situation. Could be a useful gadget.

Maybe not.

Here is a picture of the diode used on Yamaha twinshocks to prevent the motor getting a spark when rotating backwards. This diode is from a 1977 TY250D