They need to kill off all the things they can't support in the transition while building features into Android they need to port over to continue to support for Chromebooks (like the Linux Terminal stuff).
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...