Automatically Add a “No Comments” Message to WordPress Posts

/**
 * Display a custom message when comments are closed
 */
function custom_comments_closed_message( $comments_open, $post_id ) {

    if ( ! $comments_open && get_post_type( $post_id ) === 'post' ) {
        add_filter(
            'comments_template',
            function () {
                echo '<p class="comments-closed-message">Comments are currently closed for this post.</p>';
                return false;
            }
        );
    }

    return $comments_open;
}

add_filter(
    'comments_open',
    'custom_comments_closed_message',
    10,
    2
);