I cancelled my flight in Jan 2021 and the flight credit was expiring this month. Since the travel was to Mexico, I was not able to rebook using the Flight Credit online.
However, what I found was that once you have a new flight booked over the phone with your Flight Credit, the reminder amount is issued as a Trip Credit with a new expiration a year from now. I was even able to change/rebook the flight once the price dropped and get additional credits as a Trip Credit.
If I were to cancel the new flight, a new Trip Credit would be issued with a new expiration date one year from now.
I thought I was going to lose the credit, but it worked out much better than expected.