Arduino - Can i use pins 0 and 1 (RX and TX) for buttons on a Pro Micro?

[datapaganism]

I have a button plate with a button matrix, 3 encoders (with buttons) and I want to add two buttons for paddles, I only have a single pin left on the board but I still need another one, can I use RX and TX pins for this?

I have tried it but I am getting ghost inputs ( might be serial transmission but there is nothing connected that uses it)

[datapaganism]

I managed to follow this guide and use pin 30 for my needs, but it would be nice for a clarification why pins 0 and 1 are how they are