Drupal 8 reorder module-priority in form_alter to get the menu into $form.

mymodule.module

/**
 * Implements hook_form_FORM_ID_alter()
 */
function mymodule_form_node_page_edit_form_alter(&$form, FormStateInterface $form_state) {
  kint($form); //Now $form['menu'] is present.
}

/**
 * Implements hook_module_implements_alter()
 */
function mymodule_module_implements_alter(&$implementations, $hook) {
  if($hook == 'form_alter' && isset($implementations['mymodule'])) {
    //Move mymodule to the end otherwise $form['menu'] is missing in mymodule_form_node_page_edit_form_alter()
    $group = $implementations['alingsas_context'];
    unset($implementations['alingsas_context']);
    $implementations['alingsas_context'] = $group;
  }
}