forgot these:

Yeah so does Matrix but that doesn’t mean everything Element adds as an extension magically becomes standardized.

Not the same way, there are standardized XEPs for XMPP. One may not support and not use them, yes.

All of them support E2EE.

Well, every time I’ve used XMPP recently I’ve used OMEMO, so there’s no practical difference. Every modern client supports it.

Source? Closest thing I could find is “Jitsi exists and uses XMPP under the hood”

Jingle - XEP-0166.