MSC: Remove query_auth federation endpoint.

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

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+# 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.
+
+## Proposal
+
+Remove:
+
+* [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.
+
+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 it seems like this was initially supported in Synapse v0.7.0. It is
+not clear at what point it became unused.
+
+## Alternatives
+
+The endpoint could be deprecated in removed in a future version of the specification.
+
+## Security considerations
+
+None.