Microcontroller - potential damage to IO pin

Usually the datasheets, in the absmax section, give an information like "-0.3 V or -10 mA, whichever occurs first", but unfortunately not in this case; therefore a proper answer is not possible, but I can give a bit of insight.

The -0.3V limit is there because for each pin there is a diode from ground to the pin, and from the pin to vdd. With 10 kOhm in series, this is a non issue; I would expect almost any IC to be able to handle at least a few mA in the protection diodes, and here you have a 10 kOhm resistor in series. You can even bump the resistor up to 100 kOhm, if I understand your circuit correctly.

To sum it up, with your series resistor you are not violating the absmax because the diode will conduct and block the voltage around -0.3 V, probably even less. With 100 uA you can sleep safe.

According to the datasheet this is indeed a violation of the absolute maximum rating, with the caveat that the current not being specified. It might be okay, but not guaranteed.

I would look into a small power monitor IC which can detect when the input voltage is about to get low and signal your MCU. These are available at a wide variety of ‘trigger’ voltages and can be simpler/cheaper than doing your own comparator circuit.