> Two XMPP clients might support completely different feature sets.

This is of course true of Matrix as well. Just because you document everything in one place doesn't mean every app will support it all. Or that every app will even want to support it all. If every app were exactly the same there would be no point in having multiple apps, after all.

> The network effects of WhatsApp/Signal/iMessage are the real enemy, not protocol design.

Absolutely.

Don't know if it's still this way, but I had the exact same experience with the XEPs a decade ago. Even message history is an extension, but at least it has broad support. File transfer was much harder to get right.

The servers (ejabberd included) also all defaulted to a simpler but less secure config, so I would've appreciated a writeup like this post back then.