> Do not know of a single XMPP client that requires a phone number
Here is the only one I know: https://quicksy.im/
Here is the only one I know: https://quicksy.im/
Another one is Kontalk, https://www.kontalk.org/.
Both XMPP clients require a phone number, and both present the phone number as a benefit in comparison with other XMPP clients without this requirement.