The GSMA published a blog post to announce the new features coming to RCS. The iPhone is getting end-to-end encryption RCS messages via the Messaging Layer Security (MLS) protocol:

When RCS messaging became available on the iPhone with the rollout of iOS 18, the technical director of the GSM Association teased that the next major milestone for the messaging standard is the addition of interoperable end-to-end encryption.

Phone and Android users will be able to exchange end-to-end encrypted (E2EE) RCS messages in the near future thanks to newly updated RCS specifications. The GSM Association announced that the latest RCS standard includes E2EE based on the Messaging Layer Security (MLS) protocol,