It didn't when last I checked, meaning the DLCs and soundtrack were impossible to purchase if you didn't already own vanilla, but I see that the OST has since been changed to a standalone item(which kind of suggests that's a "no").

If it still doesn't, that's unfortunately a pretty significant feature that the Steam copy has over GOG, however poorly implemented it is there.
I like the game a lot and would like to have the most feature-complete version, which I was hoping would be on GOG, for all the usual reasons.

Finally just bugged support about it and no, there's currently no getting Oldward on GOG.
Disappointing, but whaddayagonnado.

Is this still the case? On Steam you can choose between both versions - DE or Classic.

I'm afraid I don't know, as I haven't purchased it on GOG (yet), but haven't seen reason to believe it changed.
I'd love to be wrong, though.

I actually just bought it, finally, and was about to say.

The differences are a bit odd.
GOG: no Oldward, but is a separate ~16gb download (if you have/had both) and has cloud saves.
Steam: has Oldward, but no cloud saves and the size is (afaik) around 25gb even if Oldward, which downloads on top of DE, is deleted.

Chances are slim with the sequel in the oven, but it'd be nice if feature parity was given another once-over sometime down the line.