The plain template will go into templates/emails/plain. We will use the same file name wc-customer-my-custom-status-order.php.

wc-customer-my-custom-status-order.php

<?php
/**
 * Completed Order sent to Customer
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

echo "= " . $email_heading . " =\n\n";

echo sprintf( __( 'The order #%d has been My Custom Status. The order details:', 'woocommerce' ), $order->id ) . "\n\n";

echo "=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=\n\n";

/**
 * @hooked WC_Emails::order_details() Shows the order details table.
 * @hooked WC_Emails::order_schema_markup() Adds Schema.org markup.
 * @since 2.5.0
 */
do_action( 'woocommerce_email_order_details', $order, $sent_to_admin, $plain_text, $email );

echo "\n=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=\n\n";

/**
 * @hooked WC_Emails::order_meta() Shows order meta data.
 */
do_action( 'woocommerce_email_order_meta', $order, $sent_to_admin, $plain_text, $email );

/**
 * @hooked WC_Emails::customer_details() Shows customer details
 * @hooked WC_Emails::email_address() Shows email address
 */
do_action( 'woocommerce_email_customer_details', $order, $sent_to_admin, $plain_text, $email );

echo "\n=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=\n\n";

echo apply_filters( 'woocommerce_email_footer_text', get_option( 'woocommerce_email_footer_text' ) );