Quoting mongodb/mongo-c-driver/#1795:

why is `_mongoc_client_monitor_op_killcursors()` necessary?

_mongoc_client_monitor_op_killcursors seems intended to create events for the legacy OP_KILL_CURSORS op code. But I expect _mongoc_client_monitor_op_killcursors is unused. The only call is guarded behind the presence of a namespace:

if (has_ns) {
    _mongoc_client_monitor_op_killcursors (cluster, server_stream, cursor_id, operation_id, db, collection);
}

And the caller, _mongoc_client_op_killcursors, is only called when there is no namespace:

if (db && collection) {
    _mongoc_client_killcursors_command (&client->cluster, server_stream, cursor_id, db, collection, cs);
} else {
    _mongoc_client_op_killcursors (&client->cluster, server_stream, cursor_id, operation_id, db, collection);
}

Suggest removing _mongoc_client_monitor_op_killcursors(_failed|_succeeded).