Clarify history of endpoint.

proposals/xxxx-remove-query_auth-federation-endpoint.md

@@ -1,26 +1,39 @@
 # MSCxxxx: Remove the `query_auth` federation endpoint
 
 The `query_auth` federation endpoint is unused by Synapse and should be removed.
-The current implementation in Synapse is broken and will return a 500 error in
-some situations.
+The current implementation in Synapse is not robust and will return a 500 error
+in some situations.
 
 ## Proposal
 
-Remove:
+Remove the following endpoint:
 
 * [POST `/_matrix/federation/v1/query_auth/{roomId}/{eventId}`](https://matrix.org/docs/spec/server_server/r0.1.3#post-matrix-federation-v1-query-auth-roomid-eventid)
 
 ## Potential issues
 
-Removing this endpoint impacts backwards compatibility.
+Removing this endpoint impacts backwards compatibility, in practice removing
+this endpoint should have minimal impact as it was an unused error path in
+Synapse. The federation client code to call this endpoint was removed in Synapse
+v1.5.0rc1.
 
-In practice, removing this endpoint should have minimal impact. Since 1.5.0rc1
-of Synapse this endpoint is not called (see [#6214](https://github.com/matrix-org/synapse/pull/6214)).
-During removal it was noted that the code to call this endpoint was already
-unreachable.
+Note that dendrite has never implemented this federation endpoint.
+
+### History
+
+This endpoint (and the federation client code) to call it was initially
+added in Synapse v0.7.0 (see [#43](https://github.com/matrix-org/synapse/pull/43)).
+The federation client code was heavily modified for v1.0.0rc1 (see
+[#5227](https://github.com/matrix-org/synapse/pull/5227/)),
 
-Note that it seems like this was initially supported in Synapse v0.7.0. It is
-not clear at what point it became unused.
+The federation client code to call this endpoint was removed in v1.5.0rc1 of
+Synapse (see [#6214](https://github.com/matrix-org/synapse/pull/6214). After
+that point this endpoint is not called).
+
+During removal it was noted that the code to call this endpoint was already
+unreachable. It seems that this code was never reachable and was meant for an
+error situation which was never built out (see `git log -S NOT_ANCESTOR`, the
+error condition is never assigned).
 
 ## Alternatives