EDIT: It seems like the API won’t make it after all. Work is still happening to it apparently: https://android-review.googlesource.com/q/RCS+(status:open+OR+status:merged). But it won’t be for third-party developers.
According to this (https://9to5google.com/2019/02/22/android-q-rcs-api-delay/), now there will be no developer-accessible API for RCS messages until Android R at the earliest.
There isn’t a way (short of some vendor-specific API) at the moment, but support for programatically interfacing with RCS is underway if the code commits are any indication of the direction Android is going… https://android-review.googlesource.com/c/platform/frameworks/base/+/849669
The relevant classes are still being implemented, but it looks like you’ll be relying on these (tentatively):
RcsParticipant 
RcsThread 
Rcs1To1Thread (extends RcsThread) 
RcsGroupThread (extends RcsThread) 
RcsMessage 
RcsIncomingMessage (extends RcsMessage) 
RcsOutgoingMessage (extends RcsMessage) 
RcsPart 
RcsFileTransferPart (extends RcsPart) 
RcsLocationPart (extends RcsPart) 
RcsMultiMediaPart (extends RcsPart) 
RcsTextPart (extends RcsPart) 
This code is telling:
class RcsThreadQueryHelper {
    static final String ASCENDING = "ASCENDING";
    static final String DESCENDING = "DESCENDING";
    static final String THREAD_ID = "_id";
    static final Uri THREADS_URI = Uri.parse("content://rcs/thread");
    static final Uri PARTICIPANTS_URI = Uri.parse("content://rcs/participant");
    static final String[] THREAD_PROJECTION = new String[]{THREAD_ID};
    static String buildWhereClause(RcsThreadQueryParameters queryParameters) {
        // TODO - implement
        return null;
    }
}