

No one enables it to “give Google data”, they use it to get apps to work that use it as a dependency.
Well yes of course, but you are still giving away your data, regardless of intention. And a privacy feature of GrapheneOS is that it redirects location data requests to the OS by default. Thats why you have to disable it in the settings if you want Google to have this data.
RCS is neither.
AFAIK it is specifically meant as a replacement for SMS and is used with regular pre-installed SMS Apps.
https://en.wikipedia.org/wiki/Rich_Communication_Services
I have no deeper knowledge of this topic, but to me it seems like Wikipedia is disagreeing.