XEP-0060 is supported by Prosody since a long time ago, as a component. I think you just need to enable it. See https://prosody.im/doc/pubsub, you need yet another extra subdomain and should include it to your server certificate.
Okay, I see the issue. The problem here is PubSub component in Prosody doesn't support Section 7.1.5 (a.k.a. publish-options) of XEP-0060, which affects the usability of OMEMO. Anyone has the plan to implement it?
So to allow for OMEMO with non-contacts or contacts without presence authorization we don't need publishing-options? Issue link by Xmpp Texting: FTW says it is necessary
Just announced on prosody MUC: https://github.com/iNPUTmice/omemo_all_access
> Traditionally OMEMO encrypted messages could only be exchanged after gaining mutual presence subscription due to the OMEMO key material being stored in PEP.
> XEP-0060 defines a method of changing the access model of a PEP node from presence to open. However Prosody does not yet support access models on PEP nodes.
> This module disables access control for all OMEMO PEP nodes (=all nodes in the namespace of eu.siacs.conversations.axolotl.*), giving everyone access to the OMEMO key material and allowing them to start OMEMO sessions with users on this server.