WooCommerce - Split shipping class items into a new package and limit shipping methods

functions.php

/**
 * This function loops over cart items, and moves any item with shipping class 'special-class' into a new package. 
 * The new package in this example only takes flat rate shipping.
 */
function split_special_shipping_class_items( $packages ) {
	$found_item                     = false;
	$special_class                  = 'special-class'; // edit this with the slug of your shippig class
	$new_package                    = current( $packages );
	$new_package['contents']        = array();
	$new_package['contents_cost']   = 0;
	$new_package['applied_coupons'] = array();
	$new_package['ship_via']        = array( 'flat_rate' ); // Only allow flat rate for items in special class
	
	foreach ( WC()->cart->get_cart() as $item_key => $item ) {
		// Is the product in the special class?
		if ( $item['data']->needs_shipping() && $special_class === $item['data']->get_shipping_class() ) {
			$found_item                            = true;
			$new_package['contents'][ $item_key ]  = $item;
			$new_package['contents_cost']         += $item['line_total'];

			// Remove from original package
			$packages[0]['contents_cost'] = $packages[0]['contents_cost'] - $item['line_total'];
			unset( $packages[0]['contents'][ $item_key ] );

			// If there are no items left in the previous package, remove it completely.
			if ( empty( $packages[0]['contents'] ) ) {
				unset( $packages[0] );
			}
		}
	}
	if ( $found_item ) {
	   $packages[] = $new_package;
	}
	return $packages;
}

// Hook into shipping packages filter
add_filter( 'woocommerce_cart_shipping_packages', 'split_special_shipping_class_items' );