As of SERVER-29453, it is no longer possible to remove the featureCompatibilityVersion document. Once SERVER-32205 is completed, it will no longer be possible to drop the admin database. The system.version collection cannot be dropped, since it is a system collection. Therefore, the op observer handling that changes the featureCompatibilityVersion parameter's value upon deleting the document and dropping the collection should never be reached. These op observer functions should be modified to invariant if they are reached.