diff --git a/changelog.d/11395.removal b/changelog.d/11395.removal
new file mode 100644
index 0000000000..6c1fd560ad
--- /dev/null
+++ b/changelog.d/11395.removal
@@ -0,0 +1 @@
+Remove deprecated `trust_identity_server_for_password_resets` configuration flag.
\ No newline at end of file
diff --git a/docker/conf/homeserver.yaml b/docker/conf/homeserver.yaml
index 3cba594d02..f10f78a48c 100644
--- a/docker/conf/homeserver.yaml
+++ b/docker/conf/homeserver.yaml
@@ -148,14 +148,6 @@ bcrypt_rounds: 12
 allow_guest_access: {{ "True" if SYNAPSE_ALLOW_GUEST else "False" }}
 enable_group_creation: true
 
-# The list of identity servers trusted to verify third party
-# identifiers by this server.
-#
-# Also defines the ID server which will be called when an account is
-# deactivated (one will be picked arbitrarily).
-trusted_third_party_id_servers:
-    - matrix.org
-    - vector.im
 
 ## Metrics ###
 
diff --git a/synapse/storage/databases/main/registration.py b/synapse/storage/databases/main/registration.py
index 7ee699f981..8478463a2a 100644
--- a/synapse/storage/databases/main/registration.py
+++ b/synapse/storage/databases/main/registration.py
@@ -1728,11 +1728,11 @@ class RegistrationBackgroundUpdateStore(RegistrationWorkerStore):
         )
 
         self.db_pool.updates.register_background_update_handler(
-            "user_threepids_grandfather", self._bg_user_threepids_grandfather
+            "users_set_deactivated_flag", self._background_update_set_deactivated_flag
         )
 
-        self.db_pool.updates.register_background_update_handler(
-            "users_set_deactivated_flag", self._background_update_set_deactivated_flag
+        self.db_pool.updates.register_noop_background_update(
+            "user_threepids_grandfather"
         )
 
         self.db_pool.updates.register_background_index_update(
@@ -1805,35 +1805,6 @@ class RegistrationBackgroundUpdateStore(RegistrationWorkerStore):
 
         return nb_processed
 
-    async def _bg_user_threepids_grandfather(self, progress, batch_size):
-        """We now track which identity servers a user binds their 3PID to, so
-        we need to handle the case of existing bindings where we didn't track
-        this.
-
-        We do this by grandfathering in existing user threepids assuming that
-        they used one of the server configured trusted identity servers.
-        """
-        id_servers = set(self.config.registration.trusted_third_party_id_servers)
-
-        def _bg_user_threepids_grandfather_txn(txn):
-            sql = """
-                INSERT INTO user_threepid_id_server
-                    (user_id, medium, address, id_server)
-                SELECT user_id, medium, address, ?
-                FROM user_threepids
-            """
-
-            txn.execute_batch(sql, [(id_server,) for id_server in id_servers])
-
-        if id_servers:
-            await self.db_pool.runInteraction(
-                "_bg_user_threepids_grandfather", _bg_user_threepids_grandfather_txn
-            )
-
-        await self.db_pool.updates._end_background_update("user_threepids_grandfather")
-
-        return 1
-
     async def set_user_deactivated_status(
         self, user_id: str, deactivated: bool
     ) -> None:
diff --git a/tests/utils.py b/tests/utils.py
index cf8ba5c5db..983859120f 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -119,7 +119,6 @@ def default_config(name, parse=False):
         "enable_registration": True,
         "enable_registration_captcha": False,
         "macaroon_secret_key": "not even a little secret",
-        "trusted_third_party_id_servers": [],
         "password_providers": [],
         "worker_replication_url": "",
         "worker_app": None,