hook_form_search_block_form_alter is not working with bootstrap theme?
You can try a bootstrap plugin:

themes/THEMENAME/src/Plugin/Form/SearchBlockForm.php

/**
 * @file
 * Contains \Drupal\THEMENAME\Plugin\Form\SearchBlockForm.
 */

namespace Drupal\THEMENAME\Plugin\Form;

use Drupal\bootstrap\Annotation\BootstrapForm;
use Drupal\bootstrap\Utility\Element;
use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter().
 *
 * @ingroup plugins_form
 *
 * @BootstrapForm("search_block_form")
 */
class SearchBlockForm extends \Drupal\bootstrap\Plugin\Form\SearchBlockForm {

  /**
   * {@inheritdoc}
   */
  public function alterForm(array &amp;$form, FormStateInterface $form_state, $form_id = NULL) {
    // Call the parent method from the base theme, if applicable (which it is
    // in this case because Bootstrap actually implements this alter).
    parent::alterForm($form, $form_state, $form_id);

    $form['keys']['#attributes']['placeholder'] = t('Sentence for placeholder!!!.')
  }
}


Reference:

http://drupal-bootstrap.org/api/bootstrap/docs%21plugins%21Form.md/group/plugins_form/8