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.

When Apple added RCS support to iOS 18, the company used an older version of the protocol. This caused chat messages between iPhone and Android users to be unencrypted. Fortunately, Apple has already agreed to update to the latest RCS protocol, thereby permanently fixing the issue.