There haven’t been any rumors of production issues at TSMC, so I don’t think it’s an A14 issue. TSMC has been as reliable a supplier as you could dream about.
Bleeding edge technology, available at scale, and seemingly flawless execution makes TSMC the perfect partner. To access that product, Apple apparently is willing to pay whatever TSMC has been asking. Kind of like Apple’s customers 🙂
If parts availability is the reason for the lack of a hard ship date—you may very well be right about that—I’d suspect maybe the new TouchID power button.
But more likely I think is that they simply haven’t started large scale production, and aren’t sure exactly what the ramp is going to look like yet, as suggested by
@CarlJ a couple posts above.