Question: Is the Steam version of Divinity: Original Sin DRM-free?
Answer: Yes. The Steam files for Divinity: Original Sin is confirmed to be DRM-free. You can launch that game without Steam via C:\Program Files (x86)\Steam\SteamApps\common\Divinity - Original Sin\Shipping\EoCApp.exe (assuming you installed Steam in the default location).
Please note that Steam Achievements, Steam Cloud, Steam Trading Cards and Steam's servers will be disabled if you launch Divinity: Original Sin without Steam.
Notes: If you're looking for a DRM-free copy of Divinity: Original Sin that doesn't require a client (Steam, etc.) to install, I suggest purchasing that game via GOG.com, because they provide you with a DRM-free executable file.