#19: delete_{$meta_type}_metadata

delete_{$meta_type}_metadata.php

<?php
/**
 * Keep posts' featured images
 *
 * @see delete_metadata()
 * 
 * @param bool $bool Default is null, anything else short-circuits delete_metadata().
 * @param int $object_id The post, user, or comment object id.
 * @param string $meta_key The meta key to delete.
 * @param string $meta_value The meta value to delete.
 * @param bool $delete_all Whether to delete all regardless of $object_id in delete_metadata(). Default is false.
 *
 * @return bool|null Null to continue, true for success, false for failure.
 */
function wpdocs_keep_featured_images( $bool, $object_id, $meta_key, $meta_value, $delete_all ) {
	// If user isn't an admin and is trying to remove the featured image key
	if ( ! current_user_can( 'manage_options' ) && '_thumbnail_id' == $meta_key )
		return false;

	return null;
}
add_filter( 'delete_post_metadata', 'wpdocs_keep_featured_images', 10, 5 );