diff --git a/changelog.d/13754.misc b/changelog.d/13754.misc
new file mode 100644
index 0000000000..662ee00e99
--- /dev/null
+++ b/changelog.d/13754.misc
@@ -0,0 +1 @@
+Re-type hint some collections as read-only.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 2d95b1fa24..5293fa4d0e 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -15,6 +15,7 @@ import itertools
 import logging
 from typing import (
     TYPE_CHECKING,
+    AbstractSet,
     Any,
     Collection,
     Dict,
@@ -1413,10 +1414,10 @@ class SyncHandler:
     async def _generate_sync_entry_for_device_list(
         self,
         sync_result_builder: "SyncResultBuilder",
-        newly_joined_rooms: Set[str],
-        newly_joined_or_invited_or_knocked_users: Set[str],
-        newly_left_rooms: Set[str],
-        newly_left_users: Set[str],
+        newly_joined_rooms: AbstractSet[str],
+        newly_joined_or_invited_or_knocked_users: AbstractSet[str],
+        newly_left_rooms: AbstractSet[str],
+        newly_left_users: AbstractSet[str],
     ) -> DeviceListUpdates:
         """Generate the DeviceListUpdates section of sync
 
@@ -1434,8 +1435,7 @@ class SyncHandler:
         user_id = sync_result_builder.sync_config.user.to_string()
         since_token = sync_result_builder.since_token
 
-        # We're going to mutate these fields, so lets copy them rather than
-        # assume they won't get used later.
+        # Take a copy since these fields will be mutated later.
         newly_joined_or_invited_or_knocked_users = set(
             newly_joined_or_invited_or_knocked_users
         )
@@ -1635,8 +1635,8 @@ class SyncHandler:
     async def _generate_sync_entry_for_presence(
         self,
         sync_result_builder: "SyncResultBuilder",
-        newly_joined_rooms: Set[str],
-        newly_joined_or_invited_users: Set[str],
+        newly_joined_rooms: AbstractSet[str],
+        newly_joined_or_invited_users: AbstractSet[str],
     ) -> None:
         """Generates the presence portion of the sync response. Populates the
         `sync_result_builder` with the result.
@@ -1694,7 +1694,7 @@ class SyncHandler:
         self,
         sync_result_builder: "SyncResultBuilder",
         account_data_by_room: Dict[str, Dict[str, JsonDict]],
-    ) -> Tuple[Set[str], Set[str], Set[str], Set[str]]:
+    ) -> Tuple[AbstractSet[str], AbstractSet[str], AbstractSet[str], AbstractSet[str]]:
         """Generates the rooms portion of the sync response. Populates the
         `sync_result_builder` with the result.
 
@@ -2534,7 +2534,7 @@ class SyncResultBuilder:
     archived: List[ArchivedSyncResult] = attr.Factory(list)
     to_device: List[JsonDict] = attr.Factory(list)
 
-    def calculate_user_changes(self) -> Tuple[Set[str], Set[str]]:
+    def calculate_user_changes(self) -> Tuple[AbstractSet[str], AbstractSet[str]]:
         """Work out which other users have joined or left rooms we are joined to.
 
         This data only is only useful for an incremental sync.