According to new Steam documents, Valve will be launching Steam Cloud Gaming soon with a Beta of Steam Cloud Play.
It will require developers to opt in, and they're required to support Cloud Saves (or another online save method), otherwise gamers will lose their data. Developers will continue to be paid the same way, since users still need to buy the games on Steam.
Before you get too excited though, the documents say the first service connecting with it will be NVIDIA GeForce NOW. For Linux gamers then, it means next to nothing since NVIDIA have been silent on any plans for Linux support with it. However, it's clearly early on and Valve are still building features and adding to their server capacity.
In the FAQ it does state this:
Will there be other cloud gaming services added to Steam Cloud Play?
We may add additional Steam Cloud Gaming services in the future. At that time we would reach out to you to opt your games into the new service.
Quite disappointing for us here of course. We're still somewhat expecting once Valve has tested the waters with this, and built up all the mechanics around Steam to support all of it, that they would launch their own. Don't think Valve would stay reliant on an external service for too long. The curious part is in the "How to sign up" part, which mentions how developers opting into Steam Cloud Gaming will have it "hosted by Valve" with service providers (like NVIDIA) being the additional. So that perhaps lends some credit towards a Valve service.
If we hear any more on it and / or they announced something that works with Linux, we will let you know.