'click sortable' = TRUE is now standard in views data

Drupal 7

If you wanted to allow people to enable click sorting on a table view for certain fields you had to add the flag in your views data:

/**
 * Implements hook_views_data().
 */
function example_views_data() {
  $data['example']['example_field']['field'] = array(
    'click sortable' => TRUE,
  );

  return $data;
}

Drupal 8

Now in Drupal 8 you have click sortable enabled by default, though you can disable it via:

/**
 * Implements hook_views_data().
 */
function example_views_data() {
  $data['example']['example_field']['field'] = array(
    'click sortable' => FALSE,
  );

  return $data;
}

One use case for that is, when your actual rendered data is constructed on the fly, so sorting can't be done on the sql level.