Hi, there isn't any DRM, early players misunderstood how bounty/treasure keys worked.
I don't see explanation about them anywhere, they require to be connected to Internet to work?
Bounties/Treasure hunts are an end-game content of randomly generated objectives which drop to players.
They have an expiration of 1 week in which they could be completed and can be given to other players, too. You can share them directly in the multiplayer chat with a unique code. Most people also use this discussion here. - Steam Discussion for sharing codes
So, they can drop to you in single player. But if you want to use a code from another person which dropped to them - you'll have to connect to the server to verify that it's a legit code and it'll send you the objective.
The early players raged about a similar code which we gave away as a reward ONLY to GOG players. It wasn't anything special that a player would miss, just some gold and 2-3 starting items. But they didn't like that they needed to register and go online, so that the code could be redeemed. Then we patched it and the reward from the code just drops on the ground after you finish the tutorial and select your diffuculty/outfit.