People, read the developers comments:

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. 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. On behalf of everyone at Nixxes and Sucker Punch, we can't wait for PC players to start their adventure and fight for the freedom of Tsushima! Source: https://steamcommunity.com/games/2215430/announcements/detail/4188987871078331986

They strictly say that unfortunately it requires Windows to access PSN integrated features, so the multiplayer will not work because it requires said features. The singleplayer should work though. Since Concord is completely multiplayer, it needs the PSN features that only work on Windows.