16GB RAM limit on a few years old portable machine is came from LPDDR3's max capacity. Intel CPU only supports LPDDR3 as low power memory until Ice Lake suppor

16GB RAM limit on a few years old portable machine is came from LPDDR3's max capacity. Intel CPU only supports LPDDR3 as low power memory until Ice Lake supports LPDDR4. Apple previously adopted LPDDR3 for MBP15 but they finally changed to DDR4 to increase capacity with sacrifying battery life a bit.