It actually is, just need to bear with the community controller layouts (it does not have native gamepad support I think) and people play it on a mix of medium and high settings decently enough. The “Unsupported” rating does not mean it does not work…in many cases it is a question of changing in game settings and in rare ones you may fallback to desktop mode to get a few more mods / patches and/or more tinkering: I was so proud getting 60 FPS Deadly Premonition working on my Steam Deck OLED
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Friend of mine in the office played tons of fun HellDivers 2 sessions and got very high level and challenging wises sometimes with the Deck docked to the TV and sometime straight handheld mode.