Comment by jeroenhd

Full standard: https://datatracker.ietf.org/doc/html/rfc9420

With the EU's DMA requirements coming up, this is a major candidate for a standard protocol for messenger interoperability. There's no legal requirement to support it, but implementing an existing standard that supports end-to-end encryption seems like a much cheaper and safer method than building your own.

Of course actual interoperability will depend on MIMI (https://datatracker.ietf.org/wg/mimi/about/) but this is a start.