fix extra code review comments

UPDATING.md

@@ -22,6 +22,9 @@ This file documents any backwards-incompatible changes in Superset and
 assists people when migrating to a new version.
 
 ## Next
+* [9109](https://github.com/apache/incubator-superset/pull/9109): Expire `filter_immune_slices` and 
+`filter_immune_filter_fields` to favor dashboard scoped filter metadata `filter_scopes`.
+
 * [9046](https://github.com/apache/incubator-superset/pull/9046): Replaces `can_only_access_owned_queries` by
 `all_query_access` favoring a white list approach. Since a new permission is introduced use `superset init` 
 to create and associate it by default to the `Admin` role. Note that, by default, all non `Admin` users will

superset/migrations/versions/3325d4caccc8_dashboard_scoped_filters.py

@@ -78,26 +78,27 @@ def upgrade():
             if "filter_scopes" in json_metadata:
                 continue
 
-            filter_scopes = {}
             filters = [
                 slice for slice in dashboard.slices if slice.viz_type == "filter_box"
             ]
 
             # if dashboard has filter_box
             if filters:
                 filter_scopes = convert_filter_scopes(json_metadata, filters)
-
-            json_metadata.pop("filter_immune_slices", None)
-            json_metadata.pop("filter_immune_slice_fields", None)
-            if filter_scopes:
                 json_metadata["filter_scopes"] = filter_scopes
                 logging.info(
                     f"Adding filter_scopes for dashboard {dashboard.id}: {json.dumps(filter_scopes)}"
                 )
+
+            json_metadata.pop("filter_immune_slices", None)
+            json_metadata.pop("filter_immune_slice_fields", None)
+
             if json_metadata:
                 dashboard.json_metadata = json.dumps(
                     json_metadata, indent=None, separators=(",", ":"), sort_keys=True
                 )
+            else:
+                dashboard.json_metadata = None
 
             session.merge(dashboard)
         except Exception as e:

superset/utils/dashboard_filter_scopes_converter.py

@@ -16,6 +16,7 @@
 # under the License.
 import json
 import logging
+from collections import defaultdict
 from typing import Dict, List
 
 from superset.models.slice import Slice
@@ -26,13 +27,11 @@
 def convert_filter_scopes(json_metadata: Dict, filters: List[Slice]):
     filter_scopes = {}
     immuned_by_id: List[int] = json_metadata.get("filter_immune_slices") or []
-    immuned_by_column: Dict = {}
+    immuned_by_column: Dict = defaultdict(list)
     for slice_id, columns in json_metadata.get(
         "filter_immune_slice_fields", {}
     ).items():
         for column in columns:
-            if immuned_by_column.get(column, None) is None:
-                immuned_by_column[column] = []
             immuned_by_column[column].append(int(slice_id))
 
     def add_filter_scope(filter_field, filter_id):