Want to delete some custom user data while delete any user in Drupal8
Use hook_entity_delete().

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21entity.api.php/function/hook_entity_delete/8.6.x

function mymodule_entity_delete(Drupal\Core\Entity\EntityInterface $entity) {
  if ($entity-&gt;getEntityType() == 'user') {
     ...
  }
}