STM32 Anti-tamper pin deactivation
This is covered in the description of the backup control register BKP_CR:



As you can see, if the TPE bit is not set the TAMPER pin is free to be used as general purpose IO. Only setting that bit will change the function of the pin to the tamper detection.

And the reset value of the register is 0x0, so the default value is to start as standard IO and not as tamper pin.

So under normal circumstances, you don't have to do anything special to make use of that pin.