Looking at the last two sentences in part A of the Steam Subscriber Agreement:
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The Software is licensed, not sold. Your license confers no title or ownership in the Software. To make use of the Software, you must have a Steam Account and you may be required to be running the Steam client and maintaining a connection to the Internet.
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Read that multiple times and it seems that termination of a Steam account not only locks one out of downloads but also strips your license to legally use anything previously downloaded...DRM free or not.
Good point. Steam's DRM codified.
In the case of D:OS, terminating your Steam Account, you can still play the game.
(Or so I've been told, haven't tried it.)
I do know you don't have to have steam running at all to play D:OS.
You can (only in some cases, not in others), but according to Steam that would be illegal. GOG doesn't have such restriction because they aim to be DRM-free.
