diff --git a/changelog.d/17650.removal b/changelog.d/17650.removal
new file mode 100644
index 0000000000..1238815c08
--- /dev/null
+++ b/changelog.d/17650.removal
@@ -0,0 +1 @@
+Stabilise [MSC4156](https://github.com/matrix-org/matrix-spec-proposals/pull/4156) by removing the `msc4156_enabled` config setting and defaulting it to `true`.
\ No newline at end of file
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 5d99c201a7..99185db93d 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -447,6 +447,3 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
 
         # MSC4151: Report room API (Client-Server API)
         self.msc4151_enabled: bool = experimental.get("msc4151_enabled", False)
-
-        # MSC4156: Migrate server_name to via
-        self.msc4156_enabled: bool = experimental.get("msc4156_enabled", False)
diff --git a/synapse/rest/client/knock.py b/synapse/rest/client/knock.py
index e31687fc13..d7a17e1b35 100644
--- a/synapse/rest/client/knock.py
+++ b/synapse/rest/client/knock.py
@@ -53,7 +53,6 @@ def __init__(self, hs: "HomeServer"):
         super().__init__()
         self.room_member_handler = hs.get_room_member_handler()
         self.auth = hs.get_auth()
-        self._support_via = hs.config.experimental.msc4156_enabled
 
     async def on_POST(
         self,
@@ -72,15 +71,11 @@ async def on_POST(
 
             # twisted.web.server.Request.args is incorrectly defined as Optional[Any]
             args: Dict[bytes, List[bytes]] = request.args  # type: ignore
-            remote_room_hosts = parse_strings_from_args(
-                args, "server_name", required=False
-            )
-            if self._support_via:
+            # Prefer via over server_name (deprecated with MSC4156)
+            remote_room_hosts = parse_strings_from_args(args, "via", required=False)
+            if remote_room_hosts is None:
                 remote_room_hosts = parse_strings_from_args(
-                    args,
-                    "org.matrix.msc4156.via",
-                    default=remote_room_hosts,
-                    required=False,
+                    args, "server_name", required=False
                 )
         elif RoomAlias.is_valid(room_identifier):
             handler = self.room_member_handler
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index 83f84e4998..23c909ab14 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -419,7 +419,6 @@ def __init__(self, hs: "HomeServer"):
         super().__init__(hs)
         super(ResolveRoomIdMixin, self).__init__(hs)  # ensure the Mixin is set up
         self.auth = hs.get_auth()
-        self._support_via = hs.config.experimental.msc4156_enabled
 
     def register(self, http_server: HttpServer) -> None:
         # /join/$room_identifier[/$txn_id]
@@ -437,13 +436,11 @@ async def _do(
 
         # twisted.web.server.Request.args is incorrectly defined as Optional[Any]
         args: Dict[bytes, List[bytes]] = request.args  # type: ignore
-        remote_room_hosts = parse_strings_from_args(args, "server_name", required=False)
-        if self._support_via:
+        # Prefer via over server_name (deprecated with MSC4156)
+        remote_room_hosts = parse_strings_from_args(args, "via", required=False)
+        if remote_room_hosts is None:
             remote_room_hosts = parse_strings_from_args(
-                args,
-                "org.matrix.msc4156.via",
-                default=remote_room_hosts,
-                required=False,
+                args, "server_name", required=False
             )
         room_id, remote_room_hosts = await self.resolve_room_id(
             room_identifier,