There's no indication the ChromeOS-Android merger will kill support for graphical Linux apps? Android supports graphical Linux apps as well.
Only these APIs are official, and none of them are Linux specific, rather Khronos and ISO standards, alongside Android specific native APIs.
https://developer.android.com/ndk/guides/stable_apis
Also to note that even though these APIs are enough for developing games, and are also available on GNU/Linux, Valve has to make use of Proton, because none of the studios targeting Android/Linux cares about GNU/Linux.
Chrome OS supported Steam through a VM as well, and it still does, if you manually install it.
The tech stack they used for Steam on Chrome OS is apparently identical to the still-supported Crostini feature ("Linux on Chrome OS"), just with a different VM image that has Steam pre-installed: https://chromeos.dev/en/posts/bringing-steam-to-chromeos#bor...