Please use below code in your functions.php

Register Shipped Order Status in WooCommerce

/**
     * Add custom status to order list
*/
    add_action( 'init', 'register_custom_post_status', 10 );
    function register_custom_post_status() {
        register_post_status( 'wc-shipped', array(
            'label'                     => _x( 'Shipped', 'Order status', 'woocommerce' ),
            'public'                    => true,
            'exclude_from_search'       => false,
            'show_in_admin_all_list'    => true,
            'show_in_admin_status_list' => true,
            'label_count'               => _n_noop( 'Shipped <span class="count">(%s)</span>', 'Shipped <span class="count">(%s)</span>', 'woocommerce' )
        ) );

}

/**
 * Add custom status to order page drop down
 */
add_filter( 'wc_order_statuses', 'custom_wc_order_statuses' );
function custom_wc_order_statuses( $order_statuses ) {
    $order_statuses['wc-shipped'] = _x( 'Shipped', 'Order status', 'woocommerce' );
    return $order_statuses;
}

Here is the code for ending email to custom order status 

add_action('woocommerce_order_status_changed', 'shipped_status_custom_notification', 10, 4);
function shipped_status_custom_notification( $order_id, $from_status, $to_status, $order ) {

   if( $order->has_status( 'shipped' )) {

        // Getting all WC_emails objects
        $email_notifications = WC()->mailer()->get_emails();

        // Customizing Heading and subject In the WC_email processing Order object
        $email_notifications['WC_Email_Customer_Processing_Order']->heading = __('Your Order shipped','woocommerce');
        $email_notifications['WC_Email_Customer_Processing_Order']->subject = 'Your {site_title} order shipped receipt from {order_date}';

        // Sending the customized email
        $email_notifications['WC_Email_Customer_Processing_Order']->trigger( $order_id );
    }

}

add_action( 'woocommerce_order_status_wc-shipped', array( WC(), 'send_transactional_email' ), 10, 1 );


add_filter( 'woocommerce_email_actions', 'filter_woocommerce_email_actions' );
function filter_woocommerce_email_actions( $actions ){
    $actions[] = 'woocommerce_order_status_wc-shipped';
    return $actions;
}