Still works… mostly

Although it’s old, it still works. However, as written, it expects a size and a class parameter. The small changes below remove that need.

/**
 * Shortcode for post thumbnail
 */
function post_thumbnail_shortcode($atts, $content='') {
	if(!function_exists('post_thumbnail_shortcode')) {
		return;
	}
	if(empty($atts['size'])) {
		$atts['size'] = 'thumbnail';
	}
	if(empty($atts['class'])) {
		$atts['class'] = '';
	}
	return '<span class="post_thumbnail '.$atts['class'].'">'.get_the_post_thumbnail(null,$atts['size']).'</span>';
}
function post_thumbnail($str) {
	$args = wp_parse_args($str);
	echo post_thumbnail_shortcode($args);
}
add_shortcode('post_thumbnail', 'post_thumbnail_shortcode');

https://www.ads-software.com/plugins/add-post-thumbnail-shortcode/