D8 Add class in preprocess conditionally (replaces menu_get_items)

d8_menu_get_items.php

<?php

//For nodes.
if ($node = \Drupal::routeMatch()->getParameter('node')) {
  if ($node->getType() == 'article') { // Test the node type.
    $is_video = $node->get('field_is_video')->value; // Get the field value of on of the node fields.
    $class_suffix = $is_video ? 'video' : 'no-video';
    $vars['attributes']['class'][] = Html::getClass($node->getType() . '--' . $class_suffix); //Add Class to the variables.
  }
}

//For terms.
if ($term = \Drupal::routeMatch()->getParameter('taxonomy_term')) {
    if ($term->getVocabularyId() == 'region') {
      $vars['term'] = $term;
    }
}