Don't reduce stock on renewal - Subscriptions

wcs_do_not_reduce_renewal_stock.php

<?php

function wcs_do_not_reduce_renewal_stock( $reduce_stock, $order ) {
	if ( function_exists( 'wcs_order_contains_renewal' ) && wcs_order_contains_renewal( $order ) ) { // Subscriptions v2.0+
		$reduce_stock = false;
	} elseif ( class_exists( 'WC_Subscriptions_Renewal_Order' ) && WC_Subscriptions_Renewal_Order::is_renewal( $order ) ) {
		$reduce_stock = false;
	}
	return $reduce_stock;
}
add_filter( 'woocommerce_can_reduce_order_stock', 'wcs_do_not_reduce_renewal_stock', 10, 2 );