https://support.google.com/messages/thread/229405182/your-rcs-conversations-are-now-fully-end-to-end-encrypted

"RCS is the modern industry standard for dynamic and secure messaging. And now, all of your RCS conversations in Messages by Google are end-to-end encrypted, including group chats, which keeps them private between you and the people you’re messaging.

To ensure more people benefit from this added security, RCS will now be enabled by default for new and existing users*, unless they’ve previously turned RCS off in Settings. You can always turn RCS on or off in Settings at any time."