I was searching for the same – I really have to turn these off.
I checked the code, and it’s a hooked action:
add_action( 'enable_jquery_migrate_helper_notification', array( __CLASS__, 'scheduled_event_handler' ) );
and a scheduled event:
wp_schedule_event( time() + DAY_IN_SECONDS, 'weekly', 'enable_jquery_migrate_helper_notification' );
you can remove these in your child themes functions.php with this code:
add_action('init', function() {
$timestamp = wp_next_scheduled( 'enable_jquery_migrate_helper_notification' );
wp_unschedule_event($timestamp, 'enable_jquery_migrate_helper_notification');
}, 100);