Which everyone should now know given the sticky explains this exactly… Why they did not mention the new minimum requirements table in the patch notes I am not sure.
NVIDIA® GeForce® GT 640 or AMD Radeon™ R9 M290X or better - Metal capable GPU
That is a desktop GPU from 2012 or a Laptop GPU from 2014. The fact there is such a large difference in required age points towards it being a feature level problem rather than metal itself. Especially seeing how people are reporting they can play Diablo III via Metal just fine on these systems which cannot play HotS now.
These older GPUs still support playing HotS via Windows. Which is the really nonsensical thing…