WP - Require featured image

gistfile1.txt

<?php

/**
 * Require a featured image to be set before a post can be published.
 */

add_filter( 'wp_insert_post_data', function ( $data, $postarr ) {

	$post_id              = $postarr['ID'];
	$post_status          = $data['post_status'];
	$original_post_status = $postarr['original_post_status'];

	if ( $post_id && 'publish' === $post_status && 'publish' !== $original_post_status ) {
		$post_type = get_post_type( $post_id );
		if ( post_type_supports( $post_type, 'thumbnail' ) && ! has_post_thumbnail( $post_id ) ) {
			$data['post_status'] = 'draft';
		}
	}

	return $data;

}, 10, 2 );

add_action( 'admin_notices', function () {
	$post = get_post();
	if ( 'publish' !== get_post_status( $post->ID ) && ! has_post_thumbnail( $post->ID ) ) { ?>
		<div id="message" class="error">
			<p>
				<strong><?php _e( 'Please set a Featured Image. This post cannot be published without one.' ); ?></strong>
			</p>
		</div>
	<?php
	}
} );