The developer answered "What specs do I need to run 'Do Not Use until 2020' graphics quality?" on Steam's discussion boards and said that this graphics mode had pretty much as many shadows and lighting effects applied as was possible:
It's diminishing returns; this mode turns on Shadows on many, many lights in the world (Falcor lights, laser bolts, ore extractors, lifts, plus loads of others)
Looks fantastic, but the performance hit is mindboggling.
People kept selecting this difficulty and complaining the game runs slowly ¬.¬
Others in the thread agreed that it looked great, but there was noticeably lag as the game progressed and draw distance was significantly diminished as well.
So while there are no specific specs required, a very good graphics card by current standards should allow someone to at least attempt to experience it.