remove keyId from /keys endpoints (#3938)

proposals/3938-remove-keyid-from-keys-endpoints.md

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+# MSC3938: Remove deprecated `keyId` parameters from `/keys` endpoints
+
+The `keyId` path parameter on
+[`GET /_matrix/key/v2/server/{keyId}`](https://spec.matrix.org/v1.5/server-server-api/#get_matrixkeyv2serverkeyid)
+and [`GET /_matrix/key/v2/query/{serverName}/{keyId}`](https://spec.matrix.org/v1.5/server-server-api/#get_matrixkeyv2queryservernamekeyid)
+has been deprecated since before the Matrix spec was formally versioned
+([pull request](https://github.com/matrix-org/matrix-spec-proposals/pull/1423)).
+
+The reason for deprecation was primarily that it was never implemented
+correctly: making a request with a `keyId` had the same effect as making a
+request without one.
+
+## Proposal
+
+The deprecated `keyId` path parameter should be removed from
+[`GET /_matrix/key/v2/server/{keyId}`](https://spec.matrix.org/v1.5/server-server-api/#get_matrixkeyv2serverkeyid)
+and [`GET /_matrix/key/v2/query/{serverName}/{keyId}`](https://spec.matrix.org/v1.5/server-server-api/#get_matrixkeyv2queryservernamekeyid).
+
+Furthermore, a trailing slash at the end of the endpoint path will no longer be permitted.
+
+The new endpoints will simply be `GET /_matrix/key/v2/server` and `GET
+/_matrix/key/v2/query/{serverName}` respectively, and they will return all
+available keys for the given server.
+
+## Potential issues
+
+This is a breaking change: some servers (such as Synapse, until [very
+recently](https://github.com/matrix-org/synapse/pull/14525)) may include the
+`{keyId}` in outgoing requests.
+
+