Thanks for the quick feedback.
In that issue, the last info on support of XEP-0363 is 4 years old. The only other place I found anything about support of XEP-0363 in ejabberd in Freedombox is Xep-363 and xep-359 support which says it works with conversations.
Was it activated at some time in the past with non-configurable settings and later disabled? Or is the author of the forum post confused?
Could https://wiki.debian.org/FreedomBox/Manual indicate the XEPs supposed to be activated with ejabberd?
Can I help for documentation and testing? I can also contribute with money, I love Freedombox and that feature is essential to me, none of my contacts will use XMPP without it.