user_delete and user_delete_multiple have been deprecated

Procedural wrappers user_delete and user_delete_multiple have been deprecated in Drupal 8.8.0 and will be removed in Drupal 9.0.0.

You should instead use the entity api and entity storage system to delete entities.

Before:

// Delete single.
user_delete($uid);

// Delete multiple.
user_delete_multiple($uids);

After:

// Delete single.
\Drupal::entityTypeManager()->getStorage('user')->load($uid)->delete();

// Delete multiple.
$user_storage = \Drupal::entityTypeManager()->getStorage('user');
$users = $user_storage->loadMultiple($uids);
$user_storage->delete($users);