MSC4306: expose feature in the client version (#18722)

changelog.d/18722.feature

@@ -0,0 +1 @@
+Advertise experimental support for [MSC4306](https://github.com/matrix-org/matrix-spec-proposals/pull/4306) through `/_matrix/clients/versions` if enabled.

synapse/rest/client/versions.py

@@ -177,6 +177,8 @@ class VersionsRestServlet(RestServlet):
                     "uk.tcpip.msc4133": self.config.experimental.msc4133_enabled,
                     # MSC4155: Invite filtering
                     "org.matrix.msc4155": self.config.experimental.msc4155_enabled,
+                    # MSC4306: Support for thread subscriptions
+                    "org.matrix.msc4306": self.config.experimental.msc4306_enabled,
                 },
             },
         )