I hope Apple does end up moving to HBM. Unified memory has been a huge godsend, but the low memory bandwidth is just such a killer. Even/especially on M5, where the available compute is starting to starve incredibly badly on ML workloads.

AFAIK that is initially only for the iPhone?

Apple is reportedly considering skipping the higher-end M6 chips altogether, which could potentially give enough time for higher-end M7 (in over a year) to use HBM

Considering, or being forced to consider due to the hardware crunch?

Who cares? If high-end M7 chips come out with HBM and actually compete with GPUs from this decade I'll be really happy. I'm also hoping for more than 128GB of unified memory in a laptop.