/**
 * Implements hook_ENTITY_TYPE_delete().
 */
function {{ machine_name }}_ENTITY_TYPE_delete(Drupal\Core\Entity\EntityInterface $entity) {
  // Delete the entity's entry from a fictional table of all entities.
  db_delete('example_entity')
    ->condition('type', $entity->getEntityTypeId())
    ->condition('id', $entity->id())
    ->execute();
}