Google was already asked and they said no. They want to keep their non-standard RCS, both server-side and client-side and will not share it.
Or more specifically: it's a different product ("Google Messages") that just happens to be based on RCS.
They do have some partnerships with hardware manufacturers that ship Play on their devices, and they will preload Google Messages in there as well.
In essence, it doesn't exist in AOSP, and doesn't really live side-by-side with a normal messaging app (i.e. one that only does baseband native messaging), I wouldn't be surprised if the partnerships and preloading conditions state the manufacturer can't ship their own version (I think at least Samsung had to drop their own "Samsung Messages" app as reported in one of the reviews of a foldable display phone).
In a way, RCS made no difference, and whatever Google did was mostly just to compete with Meta (both FB Messenger and WhatsApp). Fun fact: Google Messages is closer to Matrix than it is to iMessage in terms of comparable technical features.