Messenger Service

\Drupal::messenger()->addMessage('This is a custom message', 'custom');
\Drupal::messenger()->addError(t('This is an error message, red in color'));
\Drupal::messenger()->addStatus(t('This is a status message, green in color'));
\Drupal::messenger()->addWarning(t('This is a warning message, yellow in color'));

\Drupal::messenger()->addMessage('This is a regular message');
\Drupal::messenger()->addStatus('This is a status message, meaning status is OK or successful (green).');
\Drupal::messenger()->addError('This is an error message (red)');
\Drupal::messenger()->addWarning('This is a warning message (orange)');

public function submitForm(array &$form, FormStateInterface $form_state) {
  $this->logger('user')->notice('Deleted %ip', ['%ip' => $this->banIp,]);
  $this->messenger()->addStatus($this->t('The IP address %ip was deleted.', [    '%ip' => $this->banIp,  ]));
  $form_state->setRedirectUrl($this->getCancelUrl());
}