How to Add a Custom Required Field to WooCommerce Checkout (and Save to Order Meta & Emails)

// Add the field above the order notes on the classic checkout.
add_action( 'woocommerce_after_order_notes', function( $checkout ) {
  woocommerce_form_field(
    'delivery_instructions',
    array(
      'type'        => 'textarea',
      'class'       => array( 'form-row-wide' ),
      'label'       => __( 'Delivery instructions', 'my-site' ),
      'placeholder' => __( 'For example: leave with reception.', 'my-site' ),
      'required'    => true,
    ),
    $checkout->get_value( 'delivery_instructions' )
  );
} );

// Add a server-side validation rule. Browser-side "required" is not enough.
add_action( 'woocommerce_checkout_process', function() {
  $value = isset( $_POST['delivery_instructions'] )
    ? sanitize_textarea_field( wp_unslash( $_POST['delivery_instructions'] ) )
    : '';

  if ( '' === $value ) {
    wc_add_notice( __( 'Please add your delivery instructions.', 'my-site' ), 'error' );
  }
} );

// Save the submitted value to the order.
add_action( 'woocommerce_checkout_create_order', function( $order ) {
  if ( isset( $_POST['delivery_instructions'] ) ) {
    $value = sanitize_textarea_field( wp_unslash( $_POST['delivery_instructions'] ) );
    $order->update_meta_data( '_delivery_instructions', $value );
  }
}, 10, 1 );

// Show the value in the order admin and customer order details.
add_action( 'woocommerce_admin_order_data_after_shipping_address', 'my_site_show_delivery_instructions' );
add_action( 'woocommerce_order_details_after_order_table', 'my_site_show_delivery_instructions' );

function my_site_show_delivery_instructions( $order ) {
  $value = $order->get_meta( '_delivery_instructions' );

  if ( $value ) {
    echo '<p><strong>' . esc_html__( 'Delivery instructions', 'my-site' ) . ':</strong><br>' . nl2br( esc_html( $value ) ) . '</p>';
  }
}

// Add the value to all WooCommerce order emails.
add_filter( 'woocommerce_email_order_meta_fields', function( $fields, $sent_to_admin, $order ) {
  $value = $order->get_meta( '_delivery_instructions' );

  if ( $value ) {
    $fields['delivery_instructions'] = array(
      'label' => __( 'Delivery instructions', 'my-site' ),
      'value' => $value,
    );
  }

  return $fields;
}, 10, 3 );