I suspect notifications are a more difficult matter than it appears to be.
> DMLS - with the first MLS messages flowing over Matrix, we want to at least provide MLS as an option alongside Megolm for encryption. It should be radically more performant in larger rooms (logarithmic rather than linear complexity), but lacks deniability (the assurance that you cannot prove a user said something in retrospect, in order to blackmail them or similar), and is still unproven technology. We’ll aim to prove it in 2021.
Last I checked federated MLS hasn't really gone anywhere since 2019. The IETF Draft is more of a wish list than anything else. https://datatracker.ietf.org/doc/draft-ietf-mls-federation/ Example:
> Section 4.1.1 OPEN QUESTION: How ordering could be enforced in this mode?
IETF MLS is one step on a long path to messenger interoperability, e.g. Matrix plans to implement MLS. Contributors to MLS include Apple, Cisco and Facebook.
Wire does not mandate disclosure of phone number or address book contacts. Their free client has been relatively stagnant for a few years as they focused on business customers, but it was so far ahead of others on usability that it's still relatively current.