form_options_flatten() is replaced by \Drupal\Core\Form\OptGroup::flattenOptions

form_options_flatten() is a helper function for creating options suitable for HTML option groups.

This procedural function is now replaced by the flattenOptions() method in the \Drupal\Core\Form\OptGroup class.

It is called in the same way:

// Drupal 7
$flattened_options = form_options_flatten($options);

// Drupal 8
use Drupal\Core\Form\OptGroup;
$flattened_options = OptGroup::flattenOptions($options);