redactions are not relayed to clients if the original event has been deleted (eg via message retention)

This issue has been migrated from [#12603](https://github.com/matrix-org/synapse/issues/12603).

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On our mobile client, we use the redact WS (https://matrix.org/docs/spec/r0.0.1/client_server.html#put-matrix-client-api-r0-rooms-roomid-redact-eventid-txnid) when a user wants to remove one of his messages. It works fine for recent messages. But, in our matrix configuration, we use a short retention period of messages (7 days). When a user wants to delete a message older than 7 days (visible/cached on client but purged from the homeserver), it seems that the ‘redact’ WS does not trigger anything. No event of type m.room.redaction is sent to the mobile clients through the sync for the deleted event.

Could the deletion/redaction mechanism be made to work regardless of the homeserver retention period ?

Start by reading the Matrix redact client-server API referenced in the issue and tracing how the redact WebSocket request interacts with message retention and client sync. Reproduce redaction of a message purged after seven days; done means clients receive an m.room.redaction event through sync even when the original event has been deleted.

Written by the indexing model from the issue text.