AN4621 "STM32L4 ultra-low-power features overview" says (page 16):
The STM32L4A6xG, thanks to the external SMPS supply option, makes it possible to force on the Vcore pins (when the MR is in use) an external VDD12 supply. When VDD12 is forced by an external source and is higher than the output of the internal LDO, the current is taken from this external supply and the overall power efficiency is significantly improved when using an external step down DC/DC converter.
I just checked the STM32L4A6xG datasheet, and none of the packages show a VDD12 pin. So, simple question I guess: What happened to external SMPS support? Was it cut from the product?