backup/import.php
147: echo $OUTPUT->notification(get_string('warning'), 'notifywarning');

backup/util/ui/renderer.php
395: $output .= $this->output->notification($warning, 'notifywarning notifyproblem');

The above are the only two uses of notifywarning in the whole of Moodle (including in the CSS, so they don't actually do anything visually).

Presumably both should simply be "notifyproblem".