The undocumented {showMigrationEvents: true} option to $changeStreams causes oplog entries with {fromMigrate: true} to be returned. This behavior was initially introduced for Atlas Search as part of SERVER-38682. One idea for detecting when mongosync would be impacted by REP-4188 was to open a {showMigrationEvents: true} change stream and detect the range deleter running from its delete change events. Unfortunately the {fromMigrate: true} designation is not preserved between the oplog entry and the change event and so there is no way to distinguish application client deletes from range deleter deletes. While a different approach will ultimately be taken for mongosync, we should improve the behavior for the {showMigrationEvents: true} option such that it preserves more information from the underlying oplog entry.