Show Availability Stock Status in Woocommerce Email

functions.php

<?php
add_filter( 'woocommerce_display_item_meta', 'show_stock_status_in_email', 10, 3 );
function show_stock_status_in_email( $html, $item, $args ) {
    // gets the product object
    $product = $item->get_product();
    // gets stock status product
    $stock_status = $product->get_stock_status();
    // show it after the product name
    $html .= '<p style="margin:0;"><strong>(' . $stock_status . ')</strong></p>';
    return $html;
}

/* or */
 
/** Add custom stock message to order emails - 14-02-2022 **/
add_action( 'woocommerce_order_item_meta_end', 'rt_order_item_meta_end', 10, 4 );
function rt_order_item_meta_end( $item_id, $item, $order, $plain_text ) {
  $product = $item->get_product();
  // if product is on backorder and backorder is allowed (adjust accordingly to your shop setup)
  if ( $product->backorders_require_notification() && $product->is_on_backorder( $item['quantity'] ) ) {
    echo '<p style="color:#ff5349; font-size:12px;">Not in stock</p>';
  }
  // else do this
  else {
    echo '<p style="color:#83b735; font-size:12px;">In stock</p>';
  }
}