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I'm not sure if I am missing something but there doesn't seem to be any way to get the selling plans for a particular product using the storefront graphql api.
The storefront ajax API has this functionality, it has a selling_plan_groups field for the product, but the graphql api does not.
Sorry, the Storefront API is not available yet. It is on our roadmap (https://shopify.dev/concepts/apps/subscription-apps#product-roadmap), but not scheduled until H1 2022.