LPDDR4 (X) is a thing...

IIRC there was some problem with LPDDR4 having issues with sleeping power consumption that slowed Apple’s adoption of it.

That is DDR4 (no LP). MacBook Air 2020, MacBook Pro 16", and the most expensive MacBook Pro 13" configuration use LPDDR4.

I wish I could find the old HN comment I’m thinking of; there was some combination of memory size, board compatibility, chipset and ram that slowed Apple from moving from LPDDR3 to LPDDR4. Such restrictions might still apply to the older chips used in their non-flagship machines.

That is, if I could remember the specific details.

Intel CPUs don't support LPDDR4(X) until Ice Lake. So previous MacBooks stuck at LPDDR3 (then max capacity is limited to 16GB) or go to DDR4(more max capacity but more power consumption).