diff --git a/changelog.d/19430.removal b/changelog.d/19430.removal
new file mode 100644
index 0000000000..8920bccc38
--- /dev/null
+++ b/changelog.d/19430.removal
@@ -0,0 +1 @@
+Remove support for [MSC3852: Expose user agent information on Device](https://github.com/matrix-org/matrix-spec-proposals/pull/3852) as the MSC was closed.
\ No newline at end of file
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index a8c9305704..41efff1d8f 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -445,9 +445,6 @@ class ExperimentalConfig(Config):
         # MSC3848: Introduce errcodes for specific event sending failures
         self.msc3848_enabled: bool = experimental.get("msc3848_enabled", False)
 
-        # MSC3852: Expose last seen user agent field on /_matrix/client/v3/devices.
-        self.msc3852_enabled: bool = experimental.get("msc3852_enabled", False)
-
         # MSC3866: M_USER_AWAITING_APPROVAL error code
         raw_msc3866_config = experimental.get("msc3866", {})
         self.msc3866 = MSC3866Config(**raw_msc3866_config)
diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 29074f5e20..9a371651fb 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -129,7 +129,6 @@ class DeviceHandler:
         self._auth_handler = hs.get_auth_handler()
         self._account_data_handler = hs.get_account_data_handler()
         self._event_sources = hs.get_event_sources()
-        self._msc3852_enabled = hs.config.experimental.msc3852_enabled
         self._query_appservices_for_keys = (
             hs.config.experimental.msc3984_appservice_key_query
         )
diff --git a/synapse/rest/client/devices.py b/synapse/rest/client/devices.py
index b39ca6a483..4b84131d32 100644
--- a/synapse/rest/client/devices.py
+++ b/synapse/rest/client/devices.py
@@ -55,7 +55,6 @@ class DevicesRestServlet(RestServlet):
         self.hs = hs
         self.auth = hs.get_auth()
         self.device_handler = hs.get_device_handler()
-        self._msc3852_enabled = hs.config.experimental.msc3852_enabled
 
     async def on_GET(self, request: SynapseRequest) -> tuple[int, JsonDict]:
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
@@ -63,17 +62,10 @@ class DevicesRestServlet(RestServlet):
             requester.user.to_string()
         )
 
-        # If MSC3852 is disabled, then the "last_seen_user_agent" field will be
-        # removed from each device. If it is enabled, then the field name will
-        # be replaced by the unstable identifier.
-        #
-        # When MSC3852 is accepted, this block of code can just be removed to
-        # expose "last_seen_user_agent" to clients.
         for device in devices:
-            last_seen_user_agent = device["last_seen_user_agent"]
+            # This field is only for admin access and should not be exposed to clients.
+            # (MSC3852, which is closed, did propose to expose it.).
             del device["last_seen_user_agent"]
-            if self._msc3852_enabled:
-                device["org.matrix.msc3852.last_seen_user_agent"] = last_seen_user_agent
 
         return 200, {"devices": devices}
 
@@ -144,7 +136,6 @@ class DeviceRestServlet(RestServlet):
         handler = hs.get_device_handler()
         self.device_handler = handler
         self.auth_handler = hs.get_auth_handler()
-        self._msc3852_enabled = hs.config.experimental.msc3852_enabled
         self._auth_delegation_enabled = (
             hs.config.mas.enabled or hs.config.experimental.msc3861.enabled
         )
@@ -159,16 +150,9 @@ class DeviceRestServlet(RestServlet):
         if device is None:
             raise NotFoundError("No device found")
 
-        # If MSC3852 is disabled, then the "last_seen_user_agent" field will be
-        # removed from each device. If it is enabled, then the field name will
-        # be replaced by the unstable identifier.
-        #
-        # When MSC3852 is accepted, this block of code can just be removed to
-        # expose "last_seen_user_agent" to clients.
-        last_seen_user_agent = device["last_seen_user_agent"]
+        # This field is only for admin access and should not be exposed to clients.
+        # (MSC3852, which is closed, did propose to expose it.)
         del device["last_seen_user_agent"]
-        if self._msc3852_enabled:
-            device["org.matrix.msc3852.last_seen_user_agent"] = last_seen_user_agent
 
         return 200, device