I think they confirmed multi classing for the full release but not for EA.
I do not remember where exactly I read this. It was a long time ago.
The wording was a bit dodgy, but what they originally claimed is that we would get multiclassing by the end of EA, after all classes were included.
Then again, chances are plans changed since then, especially since I'm getting the vibe Larian people found themselves biting more than they could chew.
On a personal note, I'm not the biggest fan of multiclassing (and that's true in general across the board, not just in D&D) so I can't really say I'm particularly anticipating the feature.
I find it a bit too meta-gamey for my taste.
