What is a trial subscription's status in the Trial Performance report if it never converts?

A trial subscription whose trial has ended but has not converted to a paid subscription will remain in Pending status until 1) the merchant pays the invoice or 2) the subscription is manually terminated by a user on your end. 

If you have your dunning configured to leave the subscription active and invoice in past due status at the end of the dunning period, then the trial subscription will remain in Pending status indefinitely.