Right now: - drag-drop in the Outliner prevents dropping inside linked collections - drag-drop in the Outliner allows dropping inside overridden collections (should not be the case) - `Object Properties` > `Collections` panel allows to add to overridden collection (should not be the case) - `Object Properties` > `Collections` panel filters out non-local collections (so adding to linked collections is forbidden) - `bpy collection.objects.link()` allows to add to linked collections (should not be the case) - `bpy collection.objects.link()` allows to add to overridden collections (should not be the case) While this might be supported in the future for overriden collections, these cases should not be allowed atm. since objects get lost on file reload. Note: for the case of the `Object Properties` > `Collections` panel, this could be improved further to filter out overridden collections as well. Reviewers: mont29, brecht Subscribers: