Updates to menu_attributes_form_node_form_alter in Menu Attributes for D8 to render Menu Attributes in the Node Editing Form.

snip_existing_menu_link_attributes.module

/**
 * Implements hook_form_FORM_ID_alter().
 *
 * Adds menu attribute options to the node's edit menu item form.
 *
 * @see _menu_attributes_form_alter()
 */
function menu_attributes_form_node_form_alter(&$form, FormStateInterface $form_state) {
  if (isset($form['menu']['link']) && isset($form['#node']->menu)) {
    $item = $form['#node']->menu;
    _menu_attributes_form_alter($form['menu']['link'], $form['menu']['link'], $form);
  }
}

snip_progress_menu_link_attributes.module

/**
 * Implements hook_form_FORM_ID_alter().
 *
 * Adds menu attribute options to the node's edit menu item form.
 *
 * @see _menu_attributes_form_alter()
 */
function menu_attributes_form_node_form_alter(&$form, FormStateInterface $form_state) {
  $node = $form_state->getFormObject()->getEntity();
  $defaults = menu_ui_get_menu_link_defaults($node);
  if (!$defaults['entity_id'] == NULL) {
    // $item = $form['#node']->menu; // This does not work in D8
    _menu_attributes_form_alter($form, $form, $form);
  }
}

In the second snip of code, on lines 9 - 11, we acquire the menu link defaults and determine whether a menu item exists for the node via the node editing form.

In the second snip of code, on line 12 we try to acquire the form menu in order to isolate the Menu Settings portion but that's not the way the new menu api works so we'll have to acquire it using a different method.

For testing we call _menu_attributes_form_alter($form, $form, $form). Our final goal will be to attach it to the Menu Settings portion of the node editing form.