How to avoid negative voltage on MCU pin from RCV420 converter?
It's probably safe enough but datasheet says the current could be as much as -13mA which is less than the absolute maximum of 20mA for chip survival but could cause odd things to happen with the MCU.
You can simply add a Schottky diode such as BAT54 from the ADC input to ground, which solves the problem related to your question here.
You might also consider an RC filter and clamping the receiver output on the positive side, but that's outside the scope of this answer.