Print custom product variation field in order email confirmation
You should be able to get the product from the order which is available in the template.

I think that this will give you the products:

{% for order_item in order_entity.getItems %}
  {{ order_item }}// The product
  {{ order_item.FIELD_NAME.value }}// The field value
{% endfor %}


Otherwise you must add the product in a preprocess function.