For those unaware, BattlEye has recently announced that they are added support for Steam's Proton Linux compatibility layer. This allows Linux users to play some Windows games on Linux. At the moment, playing Destiny 2 is not possible as the anti-cheat gets flagged.
This new added support for Proton is on an op-in basis, and as far as I'm aware, it is simple to opt-in.
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Still nothing other than a non committal response in regards to steam deck.