Success Message After Submitting Form

In building a custom form you must keep in mind a few issues: elements, functionality, and response for best user experience

Review the following code, you can see the submit method doing few things:

1. updating logger
2. set message to user
3. redirect the response to specific page

This code is working on Drupal 7 to 8.6

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

from drupal 8.6, which become more service activation is written a bit differently

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());
}

drupal_set_message was deprecated in instead you should use the messenger service

$this->messenger() returns an instance of the messenger service, the service that replaced drupal_set_message() in Drupal 8.