MSCXXXX: Remove /v1/send_join

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

Author: Johennes

proposals/XXXX-remove-send-join-v1.md

@@ -0,0 +1,94 @@
+# MSCXXXX: Remove /v1/send_join
+
+[/_matrix/federation/v1/send_join/{roomId}/{eventId}] has been superseded by a `v2` endpoint and
+deprecated with [MSC1802] in [r0.1.4] of the federation API (2020). Given the time elapsed and the
+fact that the continued presence of the derpecated endpoint occasionally causes confusion[^1], it
+should be removed from the spec.
+
+## Proposal
+
+[/_matrix/federation/v1/send_join/{roomId}/{eventId}] is removed from the spec.
+
+## Potential issues
+
+Compatibility issues appear unlikely as the ecosystem seems to have adapted rather well.
+
+### Synapse
+
+Synapse has started supporting the `v2` endpoint for incoming requests in [v1.8.0] (January 2020).
+In the same version, Synapse has also started preferring the `v2` endpoint for outgoing requests,
+falling back to `v1` only when `v2` is unavailable.
+
+- https://github.com/matrix-org/synapse/commit/54ae52ba960df2bcc1daa76b6ef83b125ab9aac9#diff-b8a7e2066859e82776a9d0803499bfdc19707b3279e8fd43457f69f18c5cfecfL524-R548
+  (incoming)
+- https://github.com/matrix-org/synapse/commit/54ae52ba960df2bcc1daa76b6ef83b125ab9aac9#diff-3cf3f534d150a0319b7bdd81c884daff978bba8f8c603d7f023d5dd5d73e1ab1R600-R632
+  (outgoing)
+
+This behaviour is still present in the currently latest release [v1.141.0] (October 2025).
+
+- https://github.com/element-hq/synapse/blob/v1.141.0/synapse/federation/transport/server/federation.py#L422
+  (incoming)
+- https://github.com/element-hq/synapse/blob/v1.141.0/synapse/federation/federation_client.py#L1356
+  (outgoing)
+
+### Dendrite
+
+Dendrite has stopped supporting the `v1` endpoint for incoming requests in December 2019 and to this
+day only offers `v2`.
+
+- https://github.com/matrix-org/dendrite/commit/af9568ba4468eb106ec305df496fbe1319fd74eb
+
+For outgoing requests Dendrite relies on gomatrixserverlib which since June 2020 is preferring `v2`,
+falling back to `v1` only when `v2` is not available.
+
+- https://github.com/matrix-org/gomatrixserverlib/commit/5539854e4abc4ce179e991fbffaccde06b695dbc
+
+### Conduit & friends
+
+Conduit has first started to support the `v2` endpoint for incoming requests in April 2021.
+
+- https://gitlab.com/famedly/conduit/-/commit/eedac4fd9610feee34963d0bb227b354f97cd210?expanded=1
+  (see `server_server.rs`, line 1755)
+
+Support for the `v1` endpoint was added shortly afterwards in July 2021.
+
+- https://gitlab.com/famedly/conduit/-/commit/48494c946496c36b2d1f85da1e8877ac95e2d664?expanded=1
+
+As of writing, this behaviour is still present in Conduit and derivates such as tuwunel today.
+
+- https://github.com/matrix-construct/tuwunel/blob/d24986edf198cc8b37a47ed94b50a72a566c5cca/src/api/server/send_join.rs#L265
+
+When Conduit first gained support for joining federated rooms in August 2020, it used `v2`
+exclusively without falling back.
+
+- https://gitlab.com/famedly/conduit/-/commit/eedac4fd9610feee34963d0bb227b354f97cd210 (see
+  `membership.rs`, line 95)
+
+Again, this behaviour is still present in Conduit and derivates such as tuwunel today.
+
+- https://github.com/matrix-construct/tuwunel/blob/d24986edf198cc8b37a47ed94b50a72a566c5cca/src/service/membership/join.rs#L710
+
+## Alternatives
+
+None.
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+None.
+
+## Dependencies
+
+None.
+
+[^1]: See e.g. https://github.com/matrix-org/matrix-spec/issues/1514 and
+    https://github.com/matrix-org/matrix-spec/issues/1515.
+
+  [/_matrix/federation/v1/send_join/{roomId}/{eventId}]: https://spec.matrix.org/v1.16/server-server-api/#put_matrixfederationv1send_joinroomideventid
+  [MSC1802]: https://github.com/matrix-org/matrix-spec-proposals/blob/old_master/proposals/1802-standardised-federation-response-format.md
+  [r0.1.4]: https://spec.matrix.org/legacy/server_server/r0.1.4.html
+  [v1.8.0]: https://github.com/matrix-org/synapse/releases/tag/v1.8.0
+  [v1.141.0]: https://github.com/element-hq/synapse/releases/tag/v1.141.0