Magento 2 – Detect Order Status Change and Send Email

You can use the salesOrderManagementV1 to add a comment to an order email and send an email:

POST /V1/orders/{id}/comments
POST /V1/orders/{id}/emails

Some details from the code:

<route url="/V1/orders/:id/emails" method="POST">
    <service class="Magento\Sales\Api\OrderManagementInterface" method="notify"/>
    <resources>
        <resource ref="Magento_Sales::sales" />
    </resources>
</route>

You can also add a comment in that email:

<route url="/V1/orders/:id/comments" method="POST">
    <service class="Magento\Sales\Api\OrderManagementInterface" method="addComment"/>
    <resources>
        <resource ref="Magento_Sales::sales" />
    </resources>
</route>

Digging into the API code:

/**
 * Adds a comment to a specified order.
 *
 * @param int $id The order ID.
 * @param \Magento\Sales\Api\Data\OrderStatusHistoryInterface $statusHistory Status history comment.
 * @return bool
 */
public function addComment($id, \Magento\Sales\Api\Data\OrderStatusHistoryInterface $statusHistory);

/**
 * Emails a user a specified order.
 *
 * @param int $id The order ID.
 * @return bool
 */
public function notify($id);

Go to System > Transactional Emails and add a new template based on the "New Order email". You may need to do this for registered customers and guests. You can change the email text in the new template. Then choose the new template for use in System > Config > Sales Emails

If you can't see support@example.com in the "New Order email" and can only see something like {{var store_email}} then check System > Config > General > Store Email Addresses to make sure you have set that up correctly.