Implement XEP-0424: Message Retraction
Implement XEP-0424: Message Retraction. Needs python-nbxmpp#138 (closed)
From the XEP:
A chat participant might want to retract a message which they've already sent out, for example if they've mistakenly sent it to the wrong recipient or groupchat.
Due to the federated and extensible nature of XMPP it's not possible to remove a message with full certainty and a retraction can only be considered an unenforceable request for such removal. Clients which don't support message retraction are not obligated to enforce the request and people could have seen or copied the message contents already.
Gajim already supports XEP-0425: Message Moderation for group chat moderation.