So, does that mean that maybe the key expired? Same difference. Revoked or expired, the point is, there was a lack of clarity as to whether you had to actually redeem the key on Steam by the specified date, or whether you could just reveal and copy it elsewhere and give it to someone much later. We still haven't gotten any confirmation on that yet.

I argued that in the past, when Humble says "disappear", they only mean from your Humble account, but that if you recorded the key somewhere, you could still use it after that date. Other people said nope, redeem it on Steam or it gets revoked/expires/whatever.

Valve definitively has a system for revoking keys that doesn't affect already used one. Remember the recent issue where a game (sorry, I can't remember which game it was) was removed from a Bundlestars bundle and all unused keys where disabled, but already used ones weren't revoked, and it was done very fast.

I'd say the reason the keys have a time limit is so that people can't stockpile them to resell them.

ShakeDown Racing and Star Ruler 2 have recently invalidated only unused keys in what seems to be a somewhat new method for disabling Steam keys.

Valve's new tool simply turns all keys to 'used' status; Trying to use any key from that batch then shows the 'duplicate key' error, whether it was ever activated or not.

I don't know if Tropico 4 has or will be voided like this, but seems likely.