The fact that they are starting to put resources behind Proton instead of Ubuntu/SteamOS is in fact a clear indication that they have given up on games written natively for Linux.
That's still Steam bringing games to Linux though, right?
Whether or not it's native or wine doesn't change the fact that Steam is pushing into Linux. They've shifted the responsibility from pushing linux support on the developer to providing linux support themselves.
In order to attract more developers to develop on Linux or at least make it work through Proton, Valve is taking the path of least resistance. Proton seems to be a good step toward that end.
Valve's decision to no longer support Ubuntu as a first-class distribution, a year ago, probably. Though that's just one distribution, not Linux as a whole.
