Was reminded of this when reading T222409.
Afaik we don't use this anywhere. I recall it being used for testing at some point, but doesn't seem useful for that either. Can be trivially changed to either mocking the registry, or using factory. (Note: The factory function feature didn't exist when we first released the object key ability.)
The use of object discourages best practices for performance and dependency injection.
This was kept for compatibility with MediaWiki 1.17alpha. Proposing to remove in MW 1.34, so as to reduce the API surface area, reduce maintenance cost, and present a simpler interface for developers that has no fewer capabilities than today.