This isn't only a copyright issue. Nintendo can also complain that these videos create a likelihood of confusion regarding an association between Nintendo and S

This isn't only a copyright issue. Nintendo can also complain that these videos create a likelihood of confusion regarding an association between Nintendo and Steam Deck. That's a trademark issue, not copyright. Before you say, "No reasonable person would believe that Switch games running on Steam Deck are authorized by Nintendo," that's an argument that you raise when you are sued for trademark infringement. No one wants that, so they take the videos down.