MIMI: https://datatracker.ietf.org/wg/mimi/about/
(More Instant Messaging Interoperability)
(More Instant Messaging Interoperability)
Indeed MLS alone is nowhere near good enough to make systems interoperate. Here's to hoping MIMI takes significantly less of a fraction of a decade to get built that MLS took (5 years since chartered, 3 years after initial due dates).