How should a user which enters is_in_billing_retry_period be treated? Will Apple force the user to pay, and therefore I shouldn't suspend the service? Or should I suspend the service until the user has payed? There's conflicting information regarding the subject on the web which makes me unsure how to handle the situation.
According to docs, you should likely implement a grace period to improve recovery, if the value is “1” for the key is_in_billing_retry_period and the expires_date is in the past. A grace period is free or limited subscription access while a subscriber is in a billing retry state.
If the value is "0" for the key is_in_billing_retry_period, the subscription is no longer valid and you can be confident you will not be paid so you should likely suspend service and ask them to check their payment information.