According to the patch notes it now runs on the Kex Engine. But, the original versions of Doom 1 and 2 are still accessible. Doom 2 is still its own game and this new game can be down patched back to the Unity version, although it would have made sense to just have this as a separate download but w/e, that's Bethesda for you.

So, I guess existing players can still run the Unity versions if they want.

Logged on just for this.