XEPs current supported are in src/XMPPThread.c, at the IQ disco advertising.

For future XEPs:
    ~ https://xmpp.org/extensions/xep-0444.html
        This allows reactions, which Matrix also has support to. The two 
        systems don't seem *too* restrictive on one-another (unlike some 
        IM platforms I won't mention), so this doesn't sound too bad.
                                HALF-IMPLEMENTED

    - https://xmpp.org/extensions/xep-0118.html
        Informations on what a user is listening to. Matrix doesn't have 
        good support for status, to be frank. Clients (including KappaChat) 
        should consider having more support for those, rather than it being 
        stuck as a FluffyChat/Nheko feature.
        If any client devs hear this, please consider adding these, 
        (especially if you're a smElement employee!)

    - https://xmpp.org/extensions/xep-0084.html
        Avatar support would be extremely useful, if just a QoL improvment.
        Matrix and XMPP both have support for these.