Seems like 2d integration is kind of a one-off that isn't going to double density every 24 months for the next 30 years, or am I misunderstanding what you mean?
The reason I say 3d stacking both is and is not a cheat is that Moore's Law is silent regarding the 3rd dimension. It only talks about transistor density within a 2d area. Counting transistors stacked above as an increase in 2d density isn't specifically ruled out in the Law, and so it's not exactly a cheat. But it sure does seem to violate the spirit of the Law as commonly understood, so it's not not a cheat either.
A 3d Moore's Law would probably focus on density of transistors within a volume, rather than an area, and so 3d stacking would not represent an improvement.
Yes, 2D integration is a one-off since we can't extend it indefinitely. 3D stacking is a one-off too. Barely legal according to the Law.
But then there's lithographic layering. I'm not sure if that's the proper term; that's how Toshiba, Samsung and others stack about 128 NAND storage cells on top of each other, all on a single die. They're planning to make much higher stacks going forward (https://semiengineering.com/3d-nand-flash-wars-begin/
), the growth seems exponential. Complex computing logic can't be stacked like this just yet but engineers may soon find a way to stack 2, 4, 8, 2^n ... transistors. (huh, but how do we keep them from melting all the time?)
Now what about that? Is it, or would it be, in tune with Moore's law? I think yes. Various tricks, not just "simple" shrinking of transistors, will help us to get more and more transistors in new phones and servers and smart toasters and everything. As long as this continues, the Law is valid. We don't need a 3D variant of it.