I wonder why Steam and others don't support extra field in keys that's fixed, but affects the generation algorithm[0]? So instead of:

  1234-5678-90AB-CDEF

you'd have

  1234-REVIEW-5678-90AB-CDEF

and for regular keys (say, with language discrimination):

  3141-PLLANG-5926-5358-9793

This way it would be clear at the point of sale whether or not you bought the kind of key that was advertised. Steam and others could spam the press to proliferate information about what "REVIEW" vs. "xxLANG" vs. "ANYVER" vs. whatever means, so buyers using reselling platforms would be educated and could push the problem back to where it was created in the first place. Similarly, sellers of keys couldn't cheat and pass review keys as regular ones.

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[0] - So that you can't derive a key of one type from the key of another type.

You can send a key out that will only unlock a Review "branch" of a steam game. The game itself can be clearly marked as a review copy and can even be a cut down copy of the game if you want.

For Void Bastards we often have multiple branches going out to different groups for different things.

This seems easy to address: when the game is first started with a review key, show a pop-up informing the player that this is a review key with certain limits.

This way, the player immediately knows he has bought an improper key, and can address this with the store it was bought. A journalist would not mind such a pop-up.

If the game stops working without warning after a few months, angry reactions are to be expected.