Is it possible to apply NotBlueprintable (or do something else with the same result) to a parent class in order to hide it from the editor?

[Boon]

The only way is to use the “Hidden” specifier in the class header. This will prevent any further Blueprints from inheriting your C++ class (tho existing blueprints will be fine).

UCLASS(BlueprintType, Hidden)
class ....

If you want to do that on an engine class, you’ll need to edit engine source.