How to remove (or meta noindex) at page /?comments_popup=

To add noindex while a request made with comments_popup query, try the following into your themes functions.php –

add_action('wp_head', 'wpse_wp_head');
function wpse_wp_head(){
    if( isset($_REQUEST['comments_popup']) && '' != $_REQUEST['comments_popup'] ){
        echo "<meta name="robots" content="noindex,follow" />\n";
    }
}