The UpdateNotifier is a callback mechanism that is used as part of the RecordStore::updateRecord() storage engine API method. It was necessary in previous versions of MongoDB to allow storage engines that do not support document-level concurrency to notify the query execution layer that a document associated with a RecordId is being mutated.

The work in SERVER-16857 is removing the delivery of update/delete notifications to the query execution system, which means that the UpdateNotifier is no longer necessary.