Excluding certain posts from WordPress search results based on custom field values

exclude_posts_search_results.php

function exclude_nonadvertorial_search($query) {
  //only run for the main query and don't run on admin pages
  if (!is_admin() && $query->is_main_query()) {
    //now check to see if you are on a search results page
    if ($query->is_search) {
      //get sponsor posts that are NOT advertorials, so we can exclude their IDs from search
      $args = array(
        //get posts of the custom post type sponsor_post
        'post_type' => 'sponsor_post',
        //get all posts
        'posts_per_page' => -1,
        //return an array of post IDs
        'fields' => 'ids',
        //now check for posts that have a sponsor_post_type that is not 'advertorial'
        'meta_query' => array(
          'relation' => 'OR',
          array(
            'key' => 'sponsor_post_type',
            'value' => 'advertorial',
            'compare' => '!='
          ),
          //some posts don't have a sponsor_post_type meta field set, so check for those too
          array(
          'key' => 'sponsor_post_type',
          'compare' => 'NOT EXISTS'
          )
        )
      );
      //now get the posts
      $excluded_ids = get_posts($args);
      //add these post IDs to the 'post__not_in' query parameter
      $query->set('post__not_in', $excluded_ids);
    }
  }
}
add_action('pre_get_posts', 'exclude_nonadvertorial_search');