For HDR to work on the OLED Deck, it has to be enabled by the software, not the Steam Deck.
My assumptions (which may very well be absolutely wrong) is that there are no (or very few) linux native programs/games that have HDR output available, so if you are using the linux version of RetroArch installed by EmuDeck, you probably won’t be able to get HDR working… Maybe if you install the Windows version from Steam and get everything working through that version of RetroArch, it might be possible
For HDR to work on the OLED Deck, it has to be enabled by the software, not the Steam Deck.
My assumptions (which may very well be absolutely wrong) is that there are no (or very few) linux native programs/games that have HDR output available, so if you are using the linux version of RetroArch installed by EmuDeck, you probably won’t be able to get HDR working… Maybe if you install the Windows version from Steam and get everything working through that version of RetroArch, it might be possible