Drupal - Show only used taxonomy terms in exposed filters

As Adrian said it was possible to change the code in the article a bit to make it suitable for Drupal 8. Here is my solution:

function MODULENAME_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  //id of the form you want to alter
  $formId = 'views-exposed-form-agenda-page-1';
  //field tags
  $select_field = 'field_tags_target_id';

  /**
   * Apply the form_alter to a specific form #id
   * the form #id can be found through inspecting the markup
   */
  if ($form['#id'] == $formId) {
    $available_terms = _get_available_terms();
    if (isset($available_terms)) { // Unset the existing list and add new one with available terms.
      unset($form[$select_field]['#options']);

      if ($form[$select_field]['#options']['All']) {
        $form[$select_field]['#options']['All'] = '- Any Term -';
      }

      foreach ($available_terms as $available_term) {
        $tid = $available_term[0];
        $name = $available_term[1];
        $form[$select_field]['#options'][$tid] = $name;
      }
    }
    else { // Unset all options except '- Any -'
      unset($form[$select_field]['#options']);
      $form[$select_field]['#options']['All'] = '- Any Term -';
    }
  }
}

function _get_available_terms() {
  //table name of tags field
  $node_tags_table = 'node__field_tags';

  $db = \Drupal::database();
  $query = $db->select($node_tags_table, 'nft');
  $query->distinct();
  $query->join('taxonomy_term_field_data', 'tname', 'tname.tid = nft.field_tags_target_id');
  $query->fields('nft', ['field_tags_target_id']);
  $query->fields('tname', ['name']);
  $result = $query->execute();

  $term_list = [];
  while ($row = $result->fetchAssoc()) {
    array_push($term_list, [$row['field_tags_target_id'], $row['name']]);
  }

  return $term_list;
}

In Drupal 8, I was able to display only tags with results with the following code. Note that I'm using Better Exposed Filters contrib module, and display my filter as a Select.

function mymodule_form_views_exposed_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form['#id'] == 'views-exposed-form-your-view-name-page') {

    if (array_key_exists('field_activities_target_id', $form)) {
      // Rewrite the default 'All' option
      if (!empty($form['field_activities_target_id']['#options']['All'])) {
        $option_default = ['All' => t('Choose a category')];
      }

      $options = $form['field_activities_target_id']['#options'];

      $connection = Database::getConnection();
      $query = $connection->select('node__field_activities', 'activities');
      $query->join('node_field_data', 'n', 'n.nid = activities.entity_id');
      $query->fields('activities', ['field_activities_target_id']);
      $query->fields('n', ['status']);
      $query->condition('activities.bundle', 'your_node_type_name');
      $query->condition('n.status', 1);
      $data = $query->distinct()->execute();

      $results = array_flip($data->fetchAll(\PDO::FETCH_COLUMN, 'field_activities_target_id'));
      $options = array_intersect_key($options, $results);

      // Rebuild the option select
      $form['field_activities_target_id']['#options'] = $option_default + $options;
    }
  }
}

This article was a great help : https://johndevman.com/only-show-options-in-a-views-exposed-filter-that-belong-to-result-set/