mw.834515: If something tells me "not YET redeemed" I expected it to still work - otherwise it would say "expired" or "no longer redeemable" or something else meaningful.
Yes, unfortunately the orders page doesn't indicate when a code has expired (you'll note clicking the Orders History menu shows many filters, but something like Expired Gifts isn't among them). Someone who doesn't mind
tinkering with CSS can manually fix this on an individual basis. The URL has since changed to .../language code/account/... but the rest still works.
Codes for games purchased as a gift essentially never expire, but those from site giveaways need to be checked manually on the Redeem page as they typically have a shelf life of about 2 weeks.
