How to display a message in Drupal 9
Messages can be displayed using Drupal 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'));