Developers (who are not subclassing SketchupExtension,) could modify their SketchupExtension objects individually by defining a singleton method override in the registrar file to prevent the frivolous message in older SketchUp versions.
This will only work for extensions with such an override:
# After creating the SketchupExtension object referenced as EXTENSION:
EXTENSION.define_singleton_method(:mark_as_uninstalled) { super; nil }