Did a small experiment to "bypass" this cap. Just made multiple copies and re-named them slot31, slot32 and so on, i just overwrite over.While in-game you can only see 30 saves, games generally save to files that are located in the Documents folder and there is nothing stopping you from moving as many saves as you like out of that folder, and back in again later when you want to use them.
Days Gone instead saves to your user folder under
\AppData\Local\BendGame\Saved\
but the principle is the same. You can have as many slot #30s (and every other slot) as you could want.