Some code that recently popped up in the Android gerrit seems to imply that the available selection of RCS-compatible messaging apps will start to get wider when Android Q drops. The code in question pertains to the skeletal structure of what seem to be APIs that involve RCS support. Google crafted RCS support for Android Messages in-house and has not shared the code it used thus far, but comments from two Googlers working on the code imply that the APIs will be available for all in the main branch of AOSP, the master Android Open Source Project, as of Android Q.
Background: The API bits in question here are not packed away into the back of the project to be played with in the shadows. They're all tagged with TODO tags, which are reportedly Google's way of signaling that something is set to either be released to the public, or to be handed out to developers. One interesting thing to note is that one of the developers mentions "other RCS services" in an existing library that's already on its way into Android Q. This could mean that we will not only see developers able to integrate RCS services into third-party apps as they please, but also that Android Q will use some low-level system tricks to work additional magic with RCS on top of the use cases already seen, mostly revolving around a sort of refined and feature-rich SMS and MMS replacement. Another bit of code comment mentions unblocking vendor implementation and testing, implying that Google may also end up helping OEMs build a wider gamut of RCS capabilities into their default messaging apps.
Impact: This is potentially a quantum leap waiting to happen for mobile messaging. Google has been hard at work for some time now trying to get carriers to adopt RCS, and this move could make it a drop-in replacement for SMS and MMS. Those two antiquated standards, having had their heyday back in the era of flip phones, are well overdue for a modern makeover. RCS allows faster and more stable transmission of messages, photos, videos, and all sorts of other data, and can also be used to allow deeper interaction between users and large entities like businesses and government offices. While handing an API out to third-party developers and opening things up for OEMs obviously won't make RCS the go-to standard of the mobile messaging space overnight, it will set the stage for adoption to skyrocket. Whether RCS coexists with SMS and MMS on consumer devices or replaces them outright is down to a number of factors resting on various entities throughout the mobile world. SMS and MMS are unlikely to die out entirely any time soon, of course; a great many apps still support them, and until RCS is adopted worldwide, they're still the only ways to contact non-RCS customers without using data, a resource that's at a premium in many places around the world. Additionally, SMS and MMS have many non-consumer use cases, so their lifespan will likely see an otherwise undue extension, much like the fax standard that's still begrudgingly used in many government offices to this day.