[dngray]

> A well-known standard in this regard is XMPP and another competing is Matrix. XMPP is much older and part of the 'official' standards by the Internet Engineering Task Force, while Matrix is younger and setting a stronger focus on what the server should do.

It's a bit disingenuous to sell XMPP as a single IETF standard (RFC 6120 only covers core) when it is not. It is many standards known as XEPs (XMPP Extension Protocols) https://xmpp.org/extensions/

They are hugely fragmented and many many clients support some but not others. (Just take a look at the case of E2EE on XMPP), some support OTR, some support OMEMO, some do both, and neither work for other channels like VOIP. If you ever do any kind of federation you're going to want strong E2EE for privacy reasons.

It's also worth noting that the Matrix developers had quite a bit of experience developing XMPP software before they undertook the Matrix project. The specification https://matrix.org/docs/spec is an maintained document with a reference client (Riot) which has helped third party developers with implementation and example.