Is there any official documentation on these JSONP endpoints?

[JanisE]

Is there any official documentation on these JSONP endpoints?

In Shopify forums someone even claims that "the Shopify API doesn't support JSONP requests" – https://ecommerce.shopify.com/c/shopify-apis-and-technology/t/front-end-api-call-cors-issue-413232.

[freakdesign]

You should expect the /cart.json storefront endpoint to be the only one that supports jsonp. Makes sense that the REST API wouldn't. If there's some more specific use case this is related to be sure to raise it.

[JanisE]

Luckily, "cart/add.json" and "cart/change.json" is working too, and it seems to be all I need for now. As long as they're working I'm good.