I feel that this is it if you wanted to make games for handheld similar to Switch or Vita, without going through the long process of getting approval on the consoles makers platforms.
Get more info here: https://store.steampowered.com/steamdeck
You can check out the Games Industry Podcast episode for more insights for game Devs and their thoughts on this on the PC Gaming Industry.
This can be a good opportunity for indie developers to get their games on steam for the platform and use it as a test case for handheld gaming.
I am not sure how Proton will work at converting Windows games to their Linux Steam OS. This may depend on the different game genres and types and engines used. I read that Destiny and Alex legends might not work on this new hand held.
One code base or project with for PC and Mobile PC. Some of the Tech backends will need to be explored in the future and depends how much they can sell.
I think for games we are getting similar to film where there is easy access to creation tools and distribution all that is left is building Brand.