Saying the Steam Deck is digital only is like saying a tower computer is digital only. That's purely false. If you can put it on a tower computer, you can put it on the Steam Deck.
All the Steam Deck, like many modern tower computers, needs for physical copies is a USB media reader.
Because the Steam Deck is just computer hardware. I can already install whatever OS I want to and Steam won't know that it's a Steam Deck anymore.