No, AFAIK Steam will not be required for D:OS. Even Steamworks is not required for local multi-player, just online. FWIW, the Steam release of DKS did not require Steam to play (though until the DRM-free patch was released did use Securom).
Welcome to the forum.
Hi Raze,
This is
REALLY good news for me.

I know that many people like Steam, that's fine, but I don't. I had to pass games like Deus Ex HR, Skyrim and Risen 2 because they required Steam.
I know that Securom was removed from DKS and I appreciated that enough to also buy a digital copy of DKS from Larian Vault.
So far Larian Studios is next to
CD Projekt RED one of my favorites developers.
Best of luck with Original Sin and Dragon Commander.
PS No hard feelings for demolishing your fortress, huh?