Currently he SellingPlan attribute is not directly available in the OrderLineItem object. However, you can still retrieve information about subscriptions from an order using the customAttributes field in OrderLineItemwhich may contain details about the subscription.
To build a headless customer account page that shows an order as a subscription, you would have to rely on these customAttributes or on your own server-side logic to track and display subscription information.
Do OrderLineItem and CartLineItem share the same custom attributes? And does that mean the custom attribute of the OrderLineItem gets CartLineItem's custom attributes automatically? If that is the case, how do we change the custom attribute if the customer let's say decides to change its delivery frequency from "every 2 weeks" to "every 4 weeks"?