Drupal set message was deprecated in Drupal 8.5 and will be removed before Drupal 9.
Time for a roundup of this new API and how to use it.
\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)');
Code Snippet
\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)');
