啊哈，我想这是个拼写错误。

没有调用任何参数posts__not_in. 它叫post__not_in.

因此，您应该将代码更改为：

function dark_delight_get_related_posts(){
    $post = get_post();

    $args = array(
            \'posts_per_page\' => 4,
            \'ignore_sticky_posts\' => true,
            \'post__not_in\' => array($post->ID), // post and not posts
            );

    $categories = get_the_category();
    if( !empty($categories) ){
        $category = array_shift( $categories );
        $args[\'tax_query\'] = array(
            array(
                \'taxonomy\' => \'category\',
                \'field\' => \'id\',
                \'terms\' => $category->term_id,
                ),
            );
    }

    return new WP_Query( $args );
}