Connecting a powered MCU to an unpowered component
Output high: you would place 3.3V at an input pin with a floating ground. Not recommended.


Input high: Assuming you mean input with pull-up enabled. Less problematic than output high, as the pull up is about 47k Ohms. Less current can flow, still not recommended.
Input low: Also not recommended. Not sure of AT MCU use pull-down resistors, but it would provide a 47k Ohm path to ground. The switched chip will have a ground through its low side ESD diodes. But current limited. It may brown out or other effects.

Similar to 2, as the optocoupler will need a pull up or pull down to the input pin. Only difference is that current would flow through the optó and not the master mcu, protecting it's pins.


Worst case, output low, you power the mcu through the master, and it could cause too much current leading to a rise in the masters low level output voltage, or burning out both pins if current is too high.

Solutions: Use a suitable, logic level P-channel mosfet on the high side of the target mcu. Or use the reset pin of the target mcu. Or use low power states with its input pin as a "enable" pin.

Or use a series resistor between both pins to limit any current between them. A 100 kohm resistor should be sufficient.