Background reading: Rich Communication Services - Wikipedia
Given that Apple just announced that they are rolling over to Google, and will be implementing RCS next year, that means that more or less 100% of smartphones will be supporting RCS. So it becomes more pressing that the Librem 5 have an RCS client.
You would want to check out whether your carrier and the carriers of people you typically communicate with support RCS - because that will determine who has access to your communication (and potentially whether any third parties have access to your communication).
The inconsistent state of end-to-end encryption with RCS looks problematic.