XEP-0384 requires a client to query PubSub nodes of a user (eg. §4.4) . When I do so using smackx-pubsub on a prosody server, the client first sends a disco iq:
<iq firstname.lastname@example.org' id='w0eGv-16' type='get'><query xmlns='http://jabber.org/protocol/disco#info' node='eu.siacs.conversations.axolotl.devicelist'></query></iq>
due to https://github.com/igniterealtime/Smack/blob/master/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/PubSubManager.java#L237
The server responds with "service-unavailable" due to
I don't know, if this is an issue with prosody or with the OMEMO spec.
Related Smack issue: SMACK-743 https://issues.igniterealtime.org/browse/SMACK-743
Should be noted that this is about mod_pep which implements a bare minimum needed for PEP.