In this beta (MacOS 26.5 beta 1) the Messages app is able to send End-to-End Encrypted RCS messages in my testing.
Prior to updating to this beta, when on MacOS 26.4, even with my iPhone being on iOS 26.5 beta 1, any message I sent on the Messages app to Android users was unencrypted. It would break out of the encrypted state that was established by my iPhone running OS 26.5 b1, any reply to them on MacOS 26.4 was un-encrypted.
I'm glad Apple is supporting more RCS features like this but I strongly wish they would support all the messaging features that were introduced in RCS version 2.7 and fully support Universal Profile 3.0, not just End-to-End Encryption. I badly want them to support features like editing or recalling sent RCS messages, true in-line replies, and proper emoji reactions of photos and messages.
I communicate with my entire extended family, employees and co-workers over RCS messages, these features are absolutely critical.