I think that information for XMPP is inaccurate. I use it for private communication. E2E encryption is on by default in Conversations, messages are removed from a server if MAM is off.
Dino, Gajim turn on OMEMO by default & even the TUI Profanity prominently displays [unencrypted] in red at the top by default nudging you to pick OMEMO, OTR, or PGP for end-to-end encryption. The protocol is generic on purpose & meant to be extended with encryption which in the case of private chat applications, is now defacto. Much in the same way, TLS isn’t required since there are application that don’t require it, but defacto, all guides for setting up a XMPP server for chatting applications will suggest TLS where some servers have options like s2s TLS required or it won’t talk to the other server.
Seems weird that there’s a big, red no even when all the defaults point in the direction yes for human-to-human chat. Much in the same way some values are wrong like apps & servers being open source when there very much are proprietary XMPP servers out there like WhatsApp & Zoom. There’s also a reason Tails OS comes with Dino (or Pidgin) & every dark web guide explains how to connect to XMPP thru Tor + OMEMO/OTR, because it can be secure & anonymous enough for criminals & whistleblowers while being lightweight & decentralized.