Currently, for extensions that add custom namespaces, if those namespaces should be immovable the only way to implement that is via the NamespaceIsMovable hook. I propose that it also be possible via extension.json when specifying namespaces, by adding a new property ismovable (defaulting to true).
The following extensions register a handler for the hook, just to say that the custom namespace added is not movable, and would benefit
- EntitySchema (for NS_ENTITYSCHEMA_JSON)
- Flow (for NS_TOPIC)
- WikiLambda (for NS_ZOBJECT)