Reading the code, it appears to me that _mongoc_write_command_execute uses OP_MSG iff the wire version is at least 6, even for unacknowledged write concerns. It calls mongoc_cluster_run_opmsg which always sets flags to 0. It doesn't set the moreToCome flag, and it always expects a server response.

Update this: set moreToCome to 1, use an implicit session even if the user passed an explicit one (ignore explicit sessions with unack'ed writes), and don't expect a server response.