SteamOS 3.5.5 has been released to the Stable channel for Deck with the following changes: Display The default color rendering for Steam Deck LCD has been adjusted to emulate the sRGB color gamut, resulting in a slightly warmer and more vibrant color appearance. Added Settings -> Adjust Display Colo...
One says a controller firmware update and the other doesn’t. Imma go with the one that says there was a firmware change over the one that doesn’t, it’s also the one that caused the prompt on my side.