This is the last microdata-schema for our Shopify themes

microdata-schema.liquid

{%- comment -%}
This snippet structures the micro-data using JSON-LD specification. Please note that for Product especially,
the schema often changes. We try to output as much info as possible, but Google may add new requirements over time,
or change the format of some info

LAST UPDATE: May 10th 2023 (we added the "hasMerchantReturnPolicy" and "shippingDetails" to include the shipping and
return policy if they have been specified as store policies).
{%- endcomment -%}

{%- if request.page_type == 'product' -%}
  {%- assign days_product_price_valid_until = 10 | times: 86400 -%}

  {%- capture main_entity_microdata -%}
    {%- assign is_valid_global_gtin_length = false -%}

    {%- if product.selected_or_first_available_variant.barcode != blank -%}
      {%- assign gtin_string_length = product.selected_or_first_available_variant.barcode | size -%}

      {%- if gtin_string_length == 8 or gtin_string_length == 12 or gtin_string_length == 13 or gtin_string_length == 14 -%}
        {%- assign is_valid_global_gtin_length = true -%}
      {%- endif -%}
    {%- endif -%}

    "@type": "Product",
    "productID": {{ product.id | json }},
    "offers": [
      {%- for variant in product.variants -%}
        {%- assign is_valid_gtin_length = false -%}

        {%- if variant.barcode != blank -%}
          {%- assign gtin_string_length = variant.barcode | size -%}

          {%- if gtin_string_length == 8 or gtin_string_length == 12 or gtin_string_length == 13 or gtin_string_length == 14 -%}
            {%- assign is_valid_gtin_length = true -%}
          {%- endif -%}
        {%- endif -%}

        {
          "@type": "Offer",
          "name": {% if product.has_only_default_variant %}{{ product.title | json }}{% else %}{{ variant.title | json }}{% endif %},
          "availability": {%- if variant.available -%}"https://schema.org/InStock"{%- elsif variant.incoming -%}"https://schema.org/BackOrder"{% else %}"https://schema.org/OutOfStock"{%- endif -%},
          "price": {{ variant.price | divided_by: 100.0 | json }},
          "priceCurrency": {{ cart.currency.iso_code | json }},
          "priceValidUntil": "{{ 'now' | date: '%s' | plus: days_product_price_valid_until | date: '%Y-%m-%d' }}",
          {%- if variant.sku != blank -%}
            "sku": {{ variant.sku | json }},
          {%- endif -%}
          {%- if variant.barcode != blank -%}
            {%- if is_valid_gtin_length -%}
              "gtin": {{ variant.barcode | json }},
            {%- else -%}
              "mpn": {{ variant.barcode | json }},
            {%- endif -%}
          {%- endif -%}
          {%- if shop.refund_policy.body != blank -%}
            "hasMerchantReturnPolicy": {
              "merchantReturnLink": {{ shop.refund_policy.url | prepend: request.origin | json }}
            },
          {%- endif -%}
          {%- if shop.shipping_policy.body != blank -%}
            "shippingDetails": {
              "shippingSettingsLink": {{ shop.shipping_policy.url | prepend: request.origin | json }}
            },
          {%- endif -%}
          "url": "{{ shop.url }}{{ product.url }}?variant={{ variant.id }}"
        }{% unless forloop.last %},{% endunless %}
      {%- endfor -%}
    ],
    {%- if product.metafields.reviews.rating.value != blank and product.metafields.reviews.rating_count.value > 0 -%}
      "aggregateRating": {
        "@type": "AggregateRating",
        "ratingValue": "{{ product.metafields.reviews.rating.value }}",
        "reviewCount": "{{ product.metafields.reviews.rating_count.value }}",
        "worstRating": "{{ product.metafields.reviews.rating.value.scale_min }}",
        "bestRating": "{{ product.metafields.reviews.rating.value.scale_max }}"
      },
    {%- endif -%}
    "brand": {
      "@type": "Brand",
      "name": {{ product.vendor | json }}
    },
    "name": {{ product.title | json }},
    "description": {{ product.description | strip_html | json }},
    "category": {{ product.type | json }},
    "url": "{{ shop.url }}{{ product.url }}",
    "sku": {{ product.selected_or_first_available_variant.sku | json }},
    {%- if product.selected_or_first_available_variant.barcode != blank -%}
      {%- if is_valid_global_gtin_length -%}
        "gtin": {{ product.selected_or_first_available_variant.barcode | json }},
      {%- else -%}
        "mpn": {{ product.selected_or_first_available_variant.barcode | json }},
      {%- endif -%}
    {%- endif -%}
    "image": {
      "@type": "ImageObject",
      "url": "https:{{ page_image | image_url: width: 1024 }}",
      "image": "https:{{ page_image | image_url: width: 1024 }}",
      "name": {{ page_image.alt | json }},
      "width": "1024",
      "height": "1024"
    }
  {%- endcapture -%}
{%- elsif request.page_type == 'article' -%}
  {%- capture main_entity_microdata -%}
    "@type": "BlogPosting",
    "mainEntityOfPage": "{{ article.url }}",
    "articleSection": {{ blog.title | json }},
    "keywords": "{{ article.tags | join: ', ' }}",
    "headline": {{ article.title | json }},
    "description": {{ article.excerpt_or_content | strip_html | truncatewords: 25 | json }},
    "dateCreated": "{{ article.created_at | date: '%Y-%m-%dT%T' }}",
    "datePublished": "{{ article.published_at | date: '%Y-%m-%dT%T' }}",
    "dateModified": "{{ article.published_at | date: '%Y-%m-%dT%T' }}",
    "image": {
      "@type": "ImageObject",
      "url": "https:{{ page_image | image_url: width: 1024 }}",
      "image": "https:{{ page_image | image_url: width: 1024 }}",
      "name": {{ page_image.alt | json }},
      "width": "1024",
      "height": "1024"
    },
    "author": {
      "@type": "Person",
      "name": "{{ article.user.first_name | escape }} {{ article.user.last_name | escape }}",
      "givenName": {{ article.user.first_name | json }},
      "familyName": {{ article.user.last_name | json }}
    },
    "publisher": {
      "@type": "Organization",
      "name": {{ shop.name | json }}
    },
    "commentCount": {{ article.comments_count }},
    "comment": [
      {%- for comment in article.comments limit: 5 -%}
        {
          "@type": "Comment",
          "author": {{ comment.author | json }},
          "datePublished": "{{ comment.created_at | date: '%Y-%m-%dT%T' }}",
          "text": {{ comment.content | json }}
        }{%- unless forloop.last -%},{%- endunless -%}
      {%- endfor -%}
    ]
  {%- endcapture -%}
{%- endif -%}

{%- capture breadcrumb_entity_microdata -%}
  "@type": "BreadcrumbList",
  "itemListElement": [{
      "@type": "ListItem",
      "position": 1,
      "name": {{ 'general.home' | t | json }},
      "item": "{{ shop.url }}"
    }

    {%- if request.page_type == 'product' -%}
      {%- if collection -%}
        ,{
          "@type": "ListItem",
          "position": 2,
          "name": {{ collection.title | json }},
          "item": "{{ shop.url }}{{ collection.url }}"
        }, {
          "@type": "ListItem",
          "position": 3,
          "name": {{ product.title | json }},
          "item": "{{ shop.url }}{{ product.url }}"
        }
      {%- else -%}
        ,{
          "@type": "ListItem",
          "position": 2,
          "name": {{ product.title | json }},
          "item": "{{ shop.url }}{{ product.url }}"
        }
      {%- endif -%}
    {%- elsif request.page_type == 'collection' -%}
        ,{
          "@type": "ListItem",
          "position": 2,
          "name": {{ collection.title | json }},
          "item": "{{ shop.url }}{{ collection.url }}"
        }
    {%- elsif request.page_type == 'blog' -%}
        ,{
          "@type": "ListItem",
          "position": 2,
          "name": {{ blog.title | json }},
          "item": "{{ shop.url }}{{ blog.url }}"
        }
    {%- elsif request.page_type == 'article' -%}
        ,{
          "@type": "ListItem",
          "position": 2,
          "name": {{ blog.title | json }},
          "item": "{{ shop.url }}{{ blog.url }}"
        }, {
          "@type": "ListItem",
          "position": 3,
          "name": {{ blog.title | json }},
          "item": "{{ shop.url }}{{ article.url }}"
        }
    {%- elsif request.page_type == 'page' -%}
       ,{
          "@type": "ListItem",
          "position": 2,
          "name": {{ page.title | json }},
          "item": "{{ shop.url }}{{ page.url }}"
        }
    {%- endif -%}
  ]
{%- endcapture -%}

{% if main_entity_microdata != blank %}
  <script type="application/ld+json">
  {
    "@context": "https://schema.org",
    {{ main_entity_microdata }}
  }
  </script>
{% endif %}

{% if breadcrumb_entity_microdata != blank %}
  <script type="application/ld+json">
  {
    "@context": "https://schema.org",
    {{ breadcrumb_entity_microdata }}
  }
  </script>
{% endif %}

{%- if request.page_type == 'index' -%}
  {%- assign potential_action_target = request.origin | append: routes.search_url | append: "?q={search_term_string}" -%}

  <script type="application/ld+json">
  [
    {
      "@context": "https://schema.org",
      "@type": "WebSite",
      "name": {{ shop.name | json }},
      "url": {{ shop.url | append: page.url | json }},
      "potentialAction": {
        "@type": "SearchAction",
        "target": {{ potential_action_target | json }},
        "query-input": "required name=search_term_string"
      }
    },
    {
      "@context": "https://schema.org",
      "@type": "Organization",
      "name": {{ shop.name | json }},
      {%- if shop.brand.logo -%}
        "logo": {{ shop.brand.logo | image_url: width: shop.brand.logo.width | prepend: "https:" | json }},
      {%- endif -%}
      {%- if shop.brand.short_description -%}
        "description": {{ shop.brand.short_description | json }},
      {%- endif -%}
      {%- if shop.brand.slogan -%}
        "slogan": {{ shop.brand.slogan | json }},
      {%- endif -%}
      {%- if shop.brand.metafields.social_links.size > 0 -%}
        "sameAs": [
          {%- for social_link in shop.brand.metafields.social_links -%}
            {{- social_link.last.value | json -}}{%- unless forloop.last -%},{%- endunless -%}
          {%- endfor -%}
        ],
      {%- endif -%}
      "url": {{ shop.url | append: page.url | json }}
    }
  ]
  </script>
{%- endif -%}

For the product.ID it seems to be working with it being as an ID. I am a bit unsure about this, while I agree it should be better, using {{ product.id | json }} is just safer in case of Shopify change in the future the format and that this format requires escaping (highly improbable but we never know).

You are right about the URL, Google does not seem to report it as an error but it should be absolute ideally. I just updated the file.