MSC2451: Remove query_auth federation endpoint.

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proposals/2451-remove-query_auth-federation-endpoint.md

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+# MSC2451: Remove the `query_auth` federation endpoint
+
+This API was added without sufficient thought nor testing. The endpoint isn't
+used in any known implementations, and we do not believe it to be necessary
+for federation to work. The only known implementation (in Synapse) was not fully
+fleshed out and is broken.
+
+For background, the idea behind this endpoint was that two homeservers would be
+able to share state events with the hope of filling in missing state from one
+of homeservers allowing state resolution to complete. This was to protect
+against a joining server not providing the full (or providing stale) state.
+
+In addition to the ideas above not coming to fruition, it is unclear whether the
+current design of this endpoint would be sufficient. If this state negotiation
+feature is needed in the future it should be redesigned from scratch via the MSC
+proposal process.
+
+## Proposal
+
+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, 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.
+
+There is no evidence of other homeserver implementations having implemented this
+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/)),
+
+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
+
+The endpoint could be deprecated and removed in a future version of the specification.
+
+## Security considerations
+
+None.