With the 2025-07 version release of the Function APIs, you can now use the cart.deliveryGroups.groupType to determine the type of delivery group. The groupType field indicates whether the delivery is a one-time purchase (ONE_TIME_PURCHASE) or part of a recurring subscription (SUBSCRIPTION).
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/use-cartdeliverygroupsgrouptype-in-function-apis-to-determine-the-type-of-delivery-group