How to change search placeholder?
This code should create the placeholder:

function mytheme_form_search_block_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  $form['keys']['#attributes']['placeholder'] = t('The new placeholder');
}


So if that does not work for you, it might be because you need to clear the cache.