Alerta de uso obrigatório de imagem destacada antes da publicação de qualquer artigo.

required-featured-image.php

<?php

     	/**
	 * Obligatory alert of to use the thumbnail before publishing any article
	 */
	function featured_image_requirement() {
		if(!has_post_thumbnail()) {

			wp_die( '<div>Your post is without thumbnail!</div><br><button class="button button-primary button-large" onclick="history.go(-1);">Back to posting</button>'); 
		} 
	}
	add_action( 'pre_post_update', 'featured_image_requirement' );
	
?>