/**
 * Implements hook_entity_predelete().
 */
function {{ machine_name }}_entity_predelete(Drupal\Core\Entity\EntityInterface $entity) {
  // Count references to this entity in a custom table before they are removed
  // upon entity deletion.
  $id = $entity->id();
  $type = $entity->getEntityTypeId();
  $count = db_select('example_entity_data')
    ->condition('type', $type)
    ->condition('id', $id)
    ->countQuery()
    ->execute()
    ->fetchField();

  // Log the count in a table that records this statistic for deleted entities.
  db_merge('example_deleted_entity_statistics')
    ->key(['type' => $type, 'id' => $id])
    ->fields(['count' => $count])
    ->execute();
}