"When commenting on a previous message, it is sometimes desired to clarify which message the comment refers to. [...] This specification defines a way to indicate that a message is a reply to a previous message. It does not intend to provide means to create sub-threads for a message."
I see this as one of the most missing features in XMPP.
As far as I can see, this feature doesn't require server support (except for unique and stable stanza IDs for groupchat messages, which ejabberd supports when MAM is enabled).
https://xmpp.org/extensions/xep-0461.html
"When commenting on a previous message, it is sometimes desired to clarify which message the comment refers to. [...] This specification defines a way to indicate that a message is a reply to a previous message. It does not intend to provide means to create sub-threads for a message."
I see this as one of the most missing features in XMPP.