How to use a product tag for adding Meta noindex?

Your close:

{% if product.tags contains 'noindex' %}

{% endif %}

The merchant doc to hide shopify pages from crawlers:

https://help.shopify.com/en/manual/promoting-marketing/seo/hide-a-page-from-search-engines

And using a metafield such as through bulk editing

https://shopify.dev/apps/marketing/seo#step-2-hide-a-resource-from-search-engines-and-sitemaps

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