E2E is not in the standard, but the Google implementation uses it.
Google added end-to-end encryption to their Messages app using the Signal Protocol as the default option for one-on-one RCS conversations starting in June 2021,[88] [89] (83] [90] In December 2022, end-to-end encryption was added to group chats in the Google Messages app for beta users and was made available to all users in August 2023.
Source: https://en.wikipedia.org/wiki/Rich_Communication_Services
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