functions.php

<?php

/**
 * SearchWP Related on specific post types
 * @see https://www.billerickson.net/code/searchwp-related-on-specific-post-types/
 */
function ea_searchwp_related_exclude( $exclude = array() ) {
	$allowed = array( 'post', 'resource' );
	$all = array_keys( get_post_types() );
	return array_diff( $all, $allowed );
}
add_filter( 'searchwp_related_excluded_post_types', 'ea_searchwp_related_exclude' );