From the FAQ i linked:

While it is highly unlikely that a government would block the whole XMPP protocol (which is used for many things), it is entirely possible that they block specific servers, or block client connections to foreign servers, as is the case with many services going through China’s Great Firewall for instance. (…) In the most extreme cases, it is possible for a network operator (or government order) to block the Jabber/XMPP network entirely. In this case, using censorship-circumvention mechanisms like Tor can help you stay in touch. However, please be aware that circumventing government censorship may be a criminal offense where you reside, and you may end up having trouble with your local authorities if they find out.

Both XMPP and Matrix can be reached via HTTPS so it becomes complicated for complete eradication. If state-level censorship is a concern of yours, Matrix is certainly more suited as a protocol as it has complex algorithms to resolve global state (consensus) in case some servers can’t talk to one another.

Some other technologies like Briar are even more suited for this threat model, as it assumes all networks are compromised and/or unreachable. That is, it relies on gossip over lan-friendly medium (local wifi, USB keys…) with optional use of Tor onion services for reaching through the Internet without exposing so much metadata (beyond the fact you’re using tor).