We've announced our plan to add RCS with Messaging Layer Security (MLS) for E2EE compatible with Google Messages and iOS. SMS/MMS will only be a fallback once that's implemented.

Google Messages is essentially the only remaining RCS client for Android and it's the only one with end-to-end encryption. It already works on GrapheneOS via a toggle for ICC authentication extending our sandboxed Google Play compatibility layer. We want to add support for it to our own Messaging app but that's not straightforward since it's not at all an open platform even to the extent of SMS/MMS.