From CRM.COM R14.0.0 the binding period becomes mandatory on conditions, which means that it is mandatory only as long as the subscription's type is not of classification Package which provides a default Binding period (it can be amended though).
From CRM.COM R18.0.0 the binding period is mandatory (Package subscription types are deprecated)
start_date
(mandatory on conditions)
Date
The date that the binding period starts.
From CRM.COM R14.0.0 onwards start_date is mandatory only if no configuration exists in Subscription Billing Term Definitions related to Binding Start Date Automation.
From CRM.COM R18.0.0 the binding period is mandatory (Package subscription types are deprecated)