Spec MSC4169: Backwards-compatible redaction sending using /send (#2298)

* Spec MSC4169: Backwards-compatible redaction sending using /send

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

* Add changelog

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

* Apply suggestions

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

---------

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

changelogs/client_server/newsfragments/2298.feature

@@ -0,0 +1 @@
+Add support for `m.room.redaction` events at the `PUT /rooms/{roomId}/send/{eventType}/{txnId}` endpoint, as per [MSC4169](https://github.com/matrix-org/matrix-spec-proposals/pull/4169).

content/client-server-api/_index.md

@@ -3315,6 +3315,14 @@ the topic to be removed from the room.
 
 #### Client behaviour
 
+{{% changed-in v="1.18" %}}
+
+If the server advertises support for a spec version that supports it, clients
+MAY use the [`PUT /rooms/{roomId}/send/{eventType}/{txnId}`](#put_matrixclientv3roomsroomidsendeventtypetxnid)
+endpoint to send `m.room.redaction` events in all room versions.
+
+They can also use the following endpoint.
+
 {{% http-api spec="client-server" api="redaction" %}}
 
 ### Forming relationships between events

data/api/client-server/room_send.yaml

@@ -20,6 +20,10 @@ paths:
   "/rooms/{roomId}/send/{eventType}/{txnId}":
     put:
       summary: Send a message event to the given room.
+      x-changedInMatrixVersion:
+        "1.18": |-
+          Homeservers must support sending `m.room.redaction` events with this endpoint
+          for all room versions.
       description: |-
         This endpoint is used to send a message event to a room. Message events
         allow access to historical events and pagination, making them suited
@@ -28,6 +32,11 @@ paths:
         The body of the request should be the content object of the event; the
         fields in this object will vary depending on the type of event. See
         [Room Events](/client-server-api/#room-events) for the m. event specification.
+
+        Homeservers MUST allow clients to send `m.room.redaction` events with this
+        endpoint for all room versions. In rooms with a version older than 11 they
+        MUST move the `redacts` property inside the `content` to the top level of
+        the event.
       operationId: sendMessage
       security:
         - accessTokenQuery: []