Do Steam keys expire...

Yup! As far as I know they never expire.

Of course not. If they "expired" that would mean you got robbed and the retailer gets sued. :P

If the game is removed from Steam?

They can expire, I remember having a Bastion key from back when it was in a humble bundle. A friend was going to buy Bastion when she saw it on sale during this past Summer sale and I told her I would just give her the key from my bundle since I never activated it. She went to use it and key was expired so I think in some cases they do.

As far as it being an actual game, I don't think they expire.

Well, when I received codes from some VGA promotions, they said the code will expire after MM/DD/YYYY

I have a spreadsheet full of real old game keys. None of them have expired (I use/give away random keys periodically to friends, never had any complaints).

I keep old keys, and gave away some without a problem. However some keys do have a time limit, and you're usually told that. When I won games on Sapphire Select Club they said they have a time limit.

Prey - a 7 year old game, and you still can activate keys for it.

Only if you leave them under light or humidity.

I'd put the text file on a flash drive and put that in a ziptop bag just to be sure. Never can be too careful about key spoilage.

You can still activate the original Half-Life :P