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.
I'm not convinced it was ever something they wholeheartedly supported -- I suspect it was more of a hedge than anything. The Steam Machine hardware project, which was their big Linux push, wound down around 2016.
Valve is actively contributing to Proton and the number of supported games increases continuously.
https://www.protondb.com/ tracks the playability of each game in Linux.
This is completely false. They've been contributing massively to the gaming scene on Linux, especially with Proton – a more fine-tuned version of Wine, that makes playing Windows games on Linux seamless and easy.
And all these improvements trickle down to the respective upstream prkjects and default version. For example Fedora 33 should use dxvk for directx games by default, all at least partially thanks to the relenteless work of Valve. :-)