Yes, you're right, matrix too. However, I've tried ruining servers for both, synapse for matrix and prosody for xmpp and I should say matrix felt very sluggish and limited, while prosody is fast and insanely flexible. In addition, client software for XMPP is more diverse and feature-rich, I'm particularly impressed by movim (web) and conversations (android). Also, there are variety of bridges for everything, e.g. matrix <-> whatsapp or xmpp <-> telegram, so one is not limited too much while committing to a certain messaging tech.