Override WordPress get_search_form with plain HTML form. Visit blog post https://bit.ly/2XX2QLQ

custom_plain_html_search_form.php

<?php

/**
 * Generate custom search form
 */
function my_search_form($form) {

    $form = '<form role="search" method="get" id="searchform" class="searchform" action="' . home_url('/') . '">
      <div>
        <label class="screen-reader-text" for="s">Search for:</label>
        <input type="search" value="' . get_search_query() . '" name="s" id="s" />
        <input type="submit" id="searchsubmit" value="Search" />
        <datalist id="search">
          <option value="WordPress"></option>
          <option value="WooCommerce"></option>
          <option value="WordPress Plugins"></option>
          <option value="WordPress Themes"></option>
          <option value="Automattic"></option>
          <option value="Widgets"></option>
          ...
        </datalist>
      </div>
    </form>';
    
    return $form;
}

add_filter('get_search_form', 'my_search_form');