Google has apparently backtracked on the idea of allowing third-party messaging apps to access RCS on Android phones. Though RCS is a public specification, APIs are needed to connect messaging apps to the RCS protocols employed by wireless network operators. No API means no RCS for outside apps.
Earlier this year it appeared that Google was prepared to offer RCS APIs within the forthcoming Android Q operating system. Right now, RCS is only available to the native Android Messages app, as well as the texting app used in many Samsung phones. RCS is a platform that expands the functionality of messages beyond just text and pictures by supporting media-rich features such as video calls, read receipts, and more. The original intent behind RCS was to bring advanced, standards-based messaging to everyone.
Developers that create alternate messaging applications, unfortunately, got their hopes up after Google added some code to the Android Open Source Project. The code implied that RCS would show up with API support in Q. That's not the case.
A new commit that appeared in the code called Hide RcsMessageStore APIs spilled the beans. The commit did as its name implies and hides the APIs. Moreover, a note was attached that read, "This feature is punted from Android Q." The reason? Testing proved it to be "infeasible."
What does this mean? Well, quite clearly there will not be any RCS APIs within Android Q. Google hasn't indicated whether the idea is being punted entirely, or simply pushed back to Android R.
It might be more useful, in the long run, were Google to provide RCS APIs within the Android Messages app itself, rather than the operating system. Perhaps Google will shed more light on the subject at I/O later this year.