Random data point for those interested regarding the use of GUCs for travel after the expiration date.
Tonight I was able to redeem my 4 GUCs (expiring Feb 2021, extended from Aug 2020 due to COVID) for a trip this summer on KLM. Used the online chat feature and was able to book and upgrade all in one shot with the agent. Expiration of the GUCs only came up at the beginning of the chat when they indicated their system showed them "closed on the cover" but once clicking into them saw they were still open. They didn't give me any issues with using them despite the travel dates being after the GUC expiration and the tickets were reissued into Z within a few minutes of them confirming the original tickets.