According to the developer:
You may notice that Steam marks the game as ‘Unsupported’ for Steam Deck. This is due to the Legends co-op multiplayer mode requiring Windows to access PlayStation Network integrated features.
They’re rolling out a new in-game PlayStation overlay that requires PSN and windows to work, and multiplayer features won’t be available without the two. It’s possible that Valve will be able to fix the windows incompatibility issue, although there’s no hope for the PSN requirement part being removed.
This is the best summary I could come up with:
Ghost of Tsushima DIRECTOR’S CUT is launching on May 16th and the developers have explained how it will work on Steam Deck but it’s a mixture of good and bad news.
Sadly on Steam Deck and presumably desktop Linux too, since they both use Proton, you’ll only be able to play single-player.
We know many of you are eager to play Ghost of Tsushima Director’s Cut on handheld gaming devices like the Steam Deck.
We’re happy to share that the single player experience, including the Iki Island expansion, can be enjoyed on Steam Deck and similar handheld gaming PCs as we’ve worked extensively to optimize performance and deliver the best possible experience on these devices.
This is due to the Legends co-op multiplayer mode requiring Windows to access PlayStation Network integrated features.
We’ve seen this sort of thing happen in the past and eventually Proton updates catch up with what’s needed.
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