Oh no, now that you pointed it out I went to check the Impaler extra key and it has vanished completely, but this is more like the cases of pre-order bonuses disappearing.

I believe it's a global phenomenon, so anyone who had the Impaler extra key and hasn't redeemed it yet is now screwed.

You can't redeem the extra key because it's now invalid or it disappeared before you wrote it down? My extra key is still there.

It disappeared completely, I remember it was in the "More" section of the game like the other extra keys, but now it's gone. Strange that this is an isolated problem, I'll have to open another ticket

I haven't seen that with extra game keys yet but I have seen it with multiplayer keys. It seems as if GOG has a pool of keys for each game that can run out. But the pool should be refilled after a while and you might even be able to speed up the process by opening a support ticket and telling them that the keys have run out.

There shouldn't be any reason to worry though, the keys should arrive sooner or later.

FYI, after sending the email, GOG Support got the missing game key for my recent purchase delivered within a day or so. I then followed up on various missing multiplayer keys and am awaiting a reply.

So the lesson to be learned here is to just email GOG Support directly and not bother with the dumb chat bot. Thanks to everyone for the responses.