None of the GPIO pins are 'multiplexed' in the sense you mean. In other words, some of the GPIO pins have alternate functions, but these are functions that you maintain control of in your configuration, and in your code. You do not need to worry about the system commandeering a GPIO pin for some other purpose.
There are many sources of detailed information about the GPIO pins available online - here's one I like - and so I'll not repeat that here.
In the interest of adhering to absolute truth, I should add this caveat to my answer: the raspberry pi is not an open-source system. There are many hardware details that remain unpublished after all of these years, and the reaspberry pi firmware is not open source. The only point I am making here is that no one without access to those closed sources can give an absolute answer to your question. I can only tell you that there are no recognized uses for GPIO pins on the RPi - i.e. if you are building a life-support system, you'd be considered reckless if you used an RPi for that. For hobby usage, yeah - no multiplexing on GPIO pins. :)