MSC4213: Remove server_name parameter from join and knock endpoints

proposals/4213-remove-server-name.md

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+# MSC4213: Remove `server_name` parameter
+
+[MSC4156] deprecated the `server_name` parameter on [`/_matrix/client/v3/join/{roomIdOrAlias}`]
+and [`/_matrix/client/v3/knock/{roomIdOrAlias}`] in favor of a new parameter `via`. This change
+shipped in [Matrix v1.12]. In line with the [deprecation policy], the `server_name` parameter
+is now eligible for removal from the spec.
+
+
+## Proposal
+
+The deprecated `server_name` parameter is removed from [`/_matrix/client/v3/join/{roomIdOrAlias}`]
+and [`/_matrix/client/v3/knock/{roomIdOrAlias}`].
+
+
+## Potential issues
+
+None. Servers can continue advertising support for earlier versions of the spec that included
+`server_name` via [`/_matrix/client/versions`].
+
+As of writing, the following stable implementations of [MSC4156] are known to the author:
+
+- synapse: https://github.com/element-hq/synapse/pull/17650
+- dendrite: https://github.com/matrix-org/dendrite/pull/3438
+- matrix-js-sdk: https://github.com/matrix-org/matrix-js-sdk/pull/4381
+- ruma: https://github.com/ruma/ruma/pull/1891
+- trixnity: https://gitlab.com/trixnity/trixnity/-/merge_requests/478
+
+
+## Alternatives
+
+None.
+
+
+## Security considerations
+
+None.
+
+
+## Unstable prefix
+
+None.
+
+
+## Dependencies
+
+None.
+
+
+[`/_matrix/client/v3/join/{roomIdOrAlias}`]: https://spec.matrix.org/v1.12/client-server-api/#post_matrixclientv3joinroomidoralias
+[`/_matrix/client/v3/knock/{roomIdOrAlias}`]: https://spec.matrix.org/v1.12/client-server-api/#post_matrixclientv3knockroomidoralias
+[`/_matrix/client/versions`]: https://spec.matrix.org/v1.10/client-server-api/#get_matrixclientversions
+[Matrix v1.12]: https://spec.matrix.org/v1.12/changelog/v1.12/
+[MSC4156]: https://github.com/matrix-org/matrix-spec-proposals/pull/4156
+[deprecation policy]: https://spec.matrix.org/v1.12/#deprecation-policy