post-class-pre-order.php

<?php

add_filter( 'post_class', function( $classes, $class = '', $post_id = '' ) {
	$product = wc_get_product( $post_id );
	if ( ! $product ) {
		return $classes;
	}

	if ( class_exists( 'WC_Pre_Orders_Product' ) && WC_Pre_Orders_Product::product_can_be_pre_ordered( $product ) ) {
		$classes[] = 'pre-order';
	}

	return $classes;
}, 10, 3 );