From what I understand, the new RCS feature is a new feature in the Google "Messages" app. "Messages" is Google's own SMS app and is the default/main sms app on Google devices like nexus and pixels phones. Owners of non-google phones can install Messages on their device, making the Messages app their new default/main sms app (their original SMS app will then no longer function when google messages is installed).
From what I also understand, Google was tired of waiting around for all the carriers to implement RCS, so google instead implemented it using its own network. So, for at least the time being, a user can only get the new RCS feature by installing google's own Messages app. But, in the future when carriers start to implement RCS in their own networks, then you won't need Messages to get this feature.
And from what I also understand, if your app is available in the playstore, then in order for your app to "send" SMS messages, it needs to be the user's default SMS app (meaning that it is used for ALL sms communications). See this link for more info on this new rule: https://www.b4x.com/android/forum/t...og-permissions-are-no-longer-available.98100/
So, if your app is the devices default/main sms app, then that would mean google's messages app is NOT installed (because both applications can not be the default sms app at the same time), so it will not effect your apps operation as long as your app is the main/default sms app.
But, if a user installs Google's Messages app (to get the RCS feature), then it will be the new main/default sms app, and since your app will no longer be the default sms app, your app will no longer work.