Add customer to the cancelled subscription email as a recipient.

customer_recipient_cancelled_subscription.php

<?php

function add_customer_to_cancelled_subscription_email( $recipient, $order ) {
    // Bail on WC settings pages since the order object isn't yet set yet
    // Not sure why this is even a thing, but shikata ga nai
    $page = $_GET['page'] = isset( $_GET['page'] ) ? $_GET['page'] : '';
    if ( 'wc-settings' === $page ) {
        return $recipient; 
    }

    // just in case
    if ( ! $order instanceof WC_Order ) {
        return $recipient; 
    }
    $items = $order->get_items();

    // check if a shipped product is in the order   
    foreach ( $items as $item ) {
        $product = $order->get_product_from_item( $item );

        // add our extra recipient if there's a shipped product - commas needed!
        // we can bail if we've found one, no need to add the recipient more than once
        if ( $product && $product->needs_shipping() ) {
            $recipient .= ', ' . $order->billing_email;
            return $recipient;
        }
    }

    return $recipient;
}
add_filter( 'woocommerce_email_recipient_cancelled_subscription', 'add_customer_to_cancelled_subscription_email', 10, 2 );