Track the Display Suite fields, that are in use on a Drupal site, to be able to remove the deprecated ones.

display_suite_list_ds_fields_in_use.php

<?php
function ds_get_field_value($key, $field, $entity, $entity_type, $bundle, $view_mode, $build = array()) {
  // …………………
  // Track the display suite fields, that are in use.
  $ds_fields = variable_get('my_active_ds_fields', array());
  $render_key = $entity_type . ' | ' . $bundle . ' | ' . $view_mode;
  if (isset($ds_fields[$key])) {
    if (!in_array($render_key, $ds_fields[$key]['used_in'])) {
      $ds_fields[$key]['used_in'][] = $render_key;
    }
  }
  else {
    $ds_fields[$key] = array(
      'field_name' => $key,
      'used_in' => array($render_key),
    );
  }
  sort($ds_fields[$key]['used_in']);
  ksort($ds_fields);
  variable_set('my_active_ds_fields', $ds_fields);
  // ………………
}