It went away after a wafer process change. Another thing that went away was the 20 MHz clock capability. Pay more, get less. Thank you, Microchip.
Well, that's bad, Why do they still have the old datasheet on their website?
2.7V to 5.5V for ATmega328PPower-down mode: 1μA at 3V
Microchip doesn't but there are lots of old ones floating around elsewhere on the web. Caveat emptor.https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-Automotive-Microcontrollers-ATmega328P_Datasheet.pdf
Note that the "automotive grade" chip is theoretically a different set of chips than the non-automotive grade chips. The "non-automotive" chips seem to still go 20MHz and have both oscillators documented. Their datasheet is combined with other ATmegaXX8 chips:https://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061B.pdf(more likely, the automotive range are the same chips, but more thoroughly tested over a more limited range of chip specs.)The "product change" that removed the full-swing oscillator was CANCELLED, and the "rev K" chips that would have lost the full swing oscillator never actually shipped.:https://media.digikey.com/pdf/PCNs/Atmel/WC154601-cancel.pdfMore discussion here: https://www.avrfreaks.net/comment/2027836#comment-2027836(The cancellation seems not to have gotten nearly the publicity that it probably deserved, given the widespread horror at the original announcement.)