Deprecated function: drupal_set_message() is deprecated

Thanks for picking this up!

diff --git a/src/Entity/Form/SiteAlertDeleteForm.php b/src/Entity/Form/SiteAlertDeleteForm.php
-    drupal_set_message($this->t('The Site Alert @label has been deleted.', ['@label' => $this->entity->label()]));
+    \Drupal::messenger()->addWarning($this->t('The Site Alert @label has been deleted.', ['@label' => $this->entity->label()]));

We shouldn't call \Drupal::messenger() in OO code. The \Drupal class is only intended to be able to call into services from legacy procedural code such as hook implementations etc. Instead we should inject the messenger service.

Luckily the entity forms already inject the messenger service so this can be easily fixed by changing the code to the following:

$this->messenger()->addWarning($this->t('The Site Alert @label has been deleted.', ['@label' => $this->entity->label()]));

Here is new patch according #4, Kindly review.

Thanks! Merged in 8.x-1.x.