Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - General Steam or DRM Free?

Yes, if you get D:OS on the Larian Vault it comes with a Steam key.

The mail was referring to the digital version of the game. All kickstarter tiers with a physical version also have a digital copy.

The game is DRM free, so if you can not connect for awhile it would just mean you could not get new updates. You would still be able to play the game.