This article does a good job of talking about how poor the windows experience on a handheld is, and why SteamOS could dominate handheld devices if windows doesn’t start to make serious changes to improve it’s useability and performance.
To be clear, I don’t want to see windows succeed here, I would absolutely love to see SteamOS take over the handheld space and be a real linux ambassador to show people that they do have other choices than windows for their computers.
No. Or rather, not yet. Valve has not released SteamOS 3 for anything other than the Steam Deck, but is planning to do so at some point in the future.
I honestly wouldn’t recommend SteamOS for anything other than a dedicated gaming device though. It excels at its use case of launching games via Steam, but for everything else it is quite cumbersome… I would much rather install a general purpose Linux distribution and run Steam on it.