Occasionally, a Multi-User Chat (XEP-0045)  room moderator or admin might wish to retract certain chat messages from the room history as part of an effort to address and remedy issues such as message spam, indecent language for the venue, exposing private third-party personal information, etc. However, as with any content moderation tool, the retraction request can only be considered as a hint and by itself can not prevent or undo any potential damage caused by the offending message, as clients which don't support message deletion are not obligated to enforce the deletion request and people could have seen or copied the message content already.
If a client or service implements message deletion, it MUST specify the 'urn:xmpp:message-retract:0' feature in its service discovery information features as specified in Service Discovery (XEP-0030)  and the Entity Capabilities profile specified in Entity Capabilities (XEP-0115) .
When a user indicates to the client that a sent message (or a received message for MUC room moderators) is meant to be retracted, the client will send a new message containing a <retract /> element with the "urn:xmpp:message-retract:0" namespace, with an id attribute set to the id of the message to be retracted.
An archiving service MAY replace the contents of a retracted message with a 'tombstone' recording the fact that the message was retracted (but otherwise preserving the fact that the message did once exist in order to aid synchronizing archives). To do so, the archiving service replaces the contents of the message with a <retracted/> element qualified by the 'urn:xmpp:message-retract:0' namespace, which SHOULD include a 'by' attribute specifying the JID of the entity that sent the retraction.
A receiving client can choose to remove the indicated message from whatever display is used for messages, from any stored history, or choose to display the fact that a message has been removed in another way.
A MUC or other service that supports message retraction SHOULD prevent further distribution of the message by the service (e.g., by not replaying the message to new occupants joining the room, omitting the message from history archive requests where possible, or replacing the original message with a 'tombstone').
A client MAY inform the user that a no-longer displayed message did previously exist and has been removed.
Clients and services MUST set the 'id' attribute on messages if they allow for message retraction.
The Sender MUST NOT send a retraction request for a message with non-messaging payloads. For example, a sender MUST NOT send a retraction for a roster item exchange request or a file transfer part.
A retraction MUST only be processed when both the original message and retraction request are received from the same full-JID (or from a JID of an appropriate admin or moderator in the case of a MUC room.)
There can never be a guarantee that a retracted message was never seen or otherwise distributed, and it is encouraged for clients and services when possible to inform users that no such guarantee exists.
When used in a Multi-User Chat (XEP-0045)  context, retractions sent by non-moderators must not be allowed (by the receiver) for messages received before the sender joined the room - particularly a full JID leaving the room then rejoining and retracting a message SHOULD be disallowed, as the entity behind the full JID in the MUC may have changed.
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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Fix the XEP title to be Message Retraction.
Use the term 'retraction' instead of 'deletion'. Added tombstone marker.