WooCommerce: Add conditional checkout fields based on products in cart

functions.php

<?php // Do not include this if already open! Code goes in theme functions.php.

/**
 * Add fields to the checkout page based on products in cart.
 *
 * @how-to        https://remicorson.com/?p=7871
 * @author        Remi Corson
 * @testedwith    WooCommerce 3.4.0
 */

add_action( 'woocommerce_checkout_fields', 'woo_add_conditional_checkout_fields' );

function woo_add_conditional_checkout_fields( $fields ) {

	foreach( WC()->cart->get_cart() as $cart_item ){
	    $product_id = $cart_item['product_id'];

			if( $product_id == 2009 ) {
				$fields['billing']['billing_field_' . $product_id] = array(
					'label'     => __('Field for Product ' . $product_id, 'woocommerce'),
					'placeholder'   => _x('Field for Product ' . $product_id, 'placeholder', 'woocommerce'),
					'required'  => false,
					'class'     => array('form-row-wide'),
					'clear'     => true
				);
			}

			if( $product_id == 2010 ) {
				$fields['billing']['billing_field_' . $product_id] = array(
					'label'     => __('Field for Product ' . $product_id, 'woocommerce'),
					'placeholder'   => _x('Field for Product ' . $product_id, 'placeholder', 'woocommerce'),
					'required'  => false,
					'class'     => array('form-row-wide'),
					'clear'     => true
				);
			}
	}

	// Return checkout fields.
	return $fields;

}