The new OP_MSG wire protocol allows the driver to tell the server not to send a response. The driver can use that capability to send unacknowledged writes to servers that support OP_MSG. That way, the driver will never send OP_INSERT/UPDATE/DELETE to servers that support OP_MSG, clearing the way for deprecation and removal of the legacy wire protocol messages.

This has to be done in MixedBulkWriteOperation as well as Insert/Update/DeleteOperation (or those can be deleted and users of them can be re-written to use MixedBulkWriteOperation)