`field_purge_field()` is described as follows:

> This function assumes all data for the field has already been purged and should only be called by [`field_purge_batch()`][1].

On Drupal, fields are effectively removed during cron tasks; in fact, `field_purge_batch()` is called from [`field_cron()`][2], the implementation of `hook_cron()` done from the Field module. There is no need for you to call neither `field_purge_field()` nor `field_purge_batch()`.

If you want to write code to purge fields for a node bundle, that code would be similar to the following one. (See the notes after the code, thought.)

      $properties = array(
        'entity_type' => 'node',
        'bundle' => 'Your bundle (a.k.a. content type)',
        'include_deleted' => TRUE,
      );
      
      $entity_manager = \Drupal::entityTypeManager();

      $fields = $entity_manager->getStorage('field_config')->loadByProperties($properties);
    
      $info = $entity_manager->getDefinitions();
      foreach ($fields as $field) {
        $entity_type = $field->getTargetEntityTypeId();
    
        // Proceed only if the module implementing the entity has not been uninstalled already.
        if (isset($info[$entity_type])) {
          $entity_manager->getStorage($entity_type)->purgeFieldData($field, 100);
          field_purge_field($field);
        }
      }

### Notes

- Purging the fields is thought to be ran during cron tasks; that is why `Drupal::entityManager()->getStorage()->purgeFieldData()` requires the batch size as argument. I used a value of 100, which would possibly allow the code to purge all the fields for that content type, but it could also cause a PHP timeout error. It would be better to run the code from PHP CLI, in order to avoid any execution time out.
- You should not need to use this code; in most of the cases, you should execute the cron tasks (which is when Drupal purges the fields of a deleted bundle), or use Drush to purge the fields, either running `hook_cron()` with it, or executing a specific Drush command (if it exists).

As for truncating the tables, that is the wrong way to proceed. It would not allow Drupal and third-party modules to delete the data they have for the fields; even if you know which tables Drupal uses, you could forget a table used by third-party modules.

  [1]: https://api.drupal.org/api/drupal/core%21modules%21field%21field.purge.inc/function/field_purge_batch/8
  [2]: https://api.drupal.org/api/drupal/core%21modules%21field%21field.module/function/field_cron/8