My Steam Deck was rendered unusable at one point because of shader cache and compatibility files filling up the 64GB internal storage completely. I was only using a 256GB microSD card at the time, as well.
As I said before, I’m pretty sure the Steam Deck doesn’t remove those files when you uninstall games, so if you install and uninstall games frequently, that internal storage fills up fast. (And that isn’t even getting into non-Steam software that only functions on the internal storage.)