When I wanted to hide some products from search I unexpectedly found that I can easily make it with the metafield "seo hidden".
So, I inspired and wanted to read something more about other available built-in metafields that could help me in future but haven't found such a list. Does this list exists?
I don't think there's an exhaustive list of metafields that Shopify uses for it's own use. Aside from the SEO ones, and Google Shopping ones though, I don't think they use many.
I have scripts which I made my own feeds with and poll all the metafields for all my products, so I have not seen many more. I don't use a lot of apps made by Shopify though.
Easy way for you to find out, is to simply poll all your metafields for your products using those API endpoints.
Fun fact I found out recently, if you look at Shopify Product Export csv you have Google fields. Those fields don't actually work with Shoify's new Google Channel, and get stored under the old deprecated Google Shopping app made by shopify. So if you upload a product csv with those fields filled out, they do nothing.