When will we have LPDDR4x memory in MBs/MBPs? Apple just announced their much expected 16" MBP, which still features DDR4 memory in order to be able to support more than 16GB RAM. I find that a bit disappointing, and am a bit puzzled as to why we still haven't seen a shift to low-power memory on macbooks. 

If my understanding is correct, Intel's newer CPUs should in theory support LPDDR4x memory, and that these should provide significant improvements in terms of power usage. 

Could someone explain what is the bottleneck in the wider adoption of this technology?
The actual "bottleneck" here is the fact that your "theory" is incorrect. 

The Intel CPUs used do not support LPDDR4 memory at all. It won't work no matter what you do. They support only DDR4 and LPDDR3 RAM. Only the 10th gen Intel mobile processors support LPDDR4 - and the new MacBook Pro has a 9th gen CPU.

The 10th gen Intel mobile processors have been announced and some models have just recently launched - they support the new LPDDR4 RAM, which is almost the only new feature. However, they're not used by Apple yet. It is logical to assume that at some point, Apple will switch to newer CPUs that support newer types of memory - but this time around was not it.