This would be especially valuable on the past usage endpoint (GET /customers/{external_customer_id}/past_usage). Closed periods may have been billed under a different plan than the customer's current one, but the usage period objects carry no plan information at all so there's no way to show which plan a historical period was billed under. Returning at least the plan_code per usage period would solve this. Currently the only theoretical link is lago_invoice_id → invoice → subscription fee, which requires N extra API calls per page (and is broken in the Python client anyway — see getlago/lago-python-client#407