Yep it’s on their blog
Valve's legal department contacted Nintendo to inquire about the announced release of Dolphin Emulator on Steam. In reply to this, a lawyer representing Nintendo of America requested Valve prevent Dolphin from releasing on the Steam store, citing the DMCA as justification. Valve then forwarded us the statement from Nintendo's lawyers, and told us that we had to come to an agreement with Nintendo in order to release on Steam. Considering the strong legal wording at the start of the document and the citation of DMCA law, we took the letter very seriously.
There doesn’t seem to be any copy protection for the Game Pass version. I just copied the game install folder from my PC to my Steam Deck, then added the .exe as a non steam game with Proton 8 and it works fine on my deck.