There are a few lists that show servers that allow free registration, here is another one: https://xmpp-servers.404.city/
Self-hosting of course recommended ;)
(Background: blabber.im closing down their servers, but the app remains available)
another more automated list: https://the-federation.info/protocol/xmpp
Extensible Messaging and Presence Protocol (XMPP) is a communication protocol for message-oriented middleware based on XML. Learn more about it here.
Also see JoinJabber.org FAQ