Host your own XMPP server and/or only connect through Tor.
İts bad option
Take a small and cheap virtual Server at hetzner.de or somwhere else, put ubuntu or debian on it. Install docker and run an fully configured and really good xmpp-service with snikket. use snikket app on android or ios to register your account.
There’s two ways to prevent leaks:
directly access a remote client over Tor: for example by running a TUI client (such as poezio/profanity) or forwarding X11 over your SSH connection to use a graphical client
isolate your client in a properly-configured system such as Whonix; a reasonably-secure OS like Qubes is usually considered the best way to do that, if you can afford the overhead of virtualization
If you’d like to take part in a study of XMPP side-channel leaks (eg. WebRTC) i’d be interested in doing just that, too.
Run tor proxy and setup xmpp client to work via the proxy.
To make sure everything work as expected you can try open page https://myip.com via the proxy
Extensible Messaging and Presence Protocol (XMPP) is a communication protocol for message-oriented middleware based on XML. Learn more about it here.
Dedicated Lemmy instance
XMPP.net Provider List
Also see JoinJabber.org FAQ