They already did that close to 8 years ago as they were preparing to launch the Steam Machine ecosystem. I don't think they have the new version of SteamOS found on the Steam Deck available for download yet, but an older version of SteamOS is still available at their site...
store.steampowered.com
Of course, the Steam Machine program kind of flopped, and SteamOS never really took off (along with Linux gaming machines in general). I suppose the compatibility situation might be better this time around though, so perhaps the Steam Deck might fare better than their prior abandoned hardware ecosystems. And if the Steam Deck manages to do reasonably well and receives continued support, perhaps SteamOS might become a more viable option for gaming systems.