I think that's changed recently. Recent release notes state it's added support for RDNA4

VKD3D-Proton gained FSR4 support and will ship in a future release of Proton, but RDNA4 requires a newer kernel.

SteamOS release notes are public at https://www.steamdeck.com/en/news, it still uses a 6.11 kernel from September 2024.