I'd guess if DD isn't on Steam by now it probably will not be, but you could check when FoV and DKS are listed (assuming they will be) and see if DD is added as well. The publishers handled the digital distribution for D2:ED, but AFAIK Larian is dealing with them directly for the upcoming releases.
For now DD is available from GOG and Impulse, and will be available directly from Larian when they bring their digital distribution store online with the release of FoV / DKS.

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