AMD

Why wouldn't it be impossible? The 3d Cache sits on top of the ordinary cache and gets data and power connections through vias embedded in the sillicon of the ordinary L3 cache. The distinction that desktop CPUs are chiplet based vs the mobile chips that are monolythic is not valid, because each chiplet is monolythic by itself and can obviously have 3d cache. So, if it suits them AMD can make 3d cache variants of their mobile chips. What's needed is for them to decide that it's worth it, then embed the vias in All the silicon (as in the Zen 3 line) and then pull the trigger sometime down the line when they decide that the market conditions are right.

The valid distinction here is that 3d cache is often very useful in server applications, while in normal PCs and laptops it benefits mostly gaming. And, the same chiplets are first binned for server... We gamers happen to enjoy this technology by accident in a way. Having in mind how bandwidth starved are all the CUs in the APUs it will be a godsend for sure. Will AMD do it, considering that they do have the vias in the chiplets in the high end GPUs and still haven't done it there... Well..