I just re-installed the latest(?) version of AVR-GCC available from the Ubuntu repository. I've also checked the 'official' avr-gcc site looking for the latest sources.
It seems that definitions for the new AVR-0, AVR-Dx, AVR-Tiny0 and AVR-Tini1 series are NOT included (not in io.h, and no device specific headers are supplied). Hell, even the not-so-new Atmega328pb and ATmega324pb are missing.
Yet these support files for these devices ARE found in the tool chain supplied with the current Arduino (1.8.13) release. You can also find the required headers on Microchip's site (if you are good with searching their site to find the right 'pacl' to download.
(Is the Microchip MC-08 compiler a re-spin of gcc?).
Why hasn't the updated sources for avr-gcc from which these tool chains are built from been spun back into the GNU hosted sources? Why haven't Debian and the other distro's updated their packages? Isn't this what is SUPPOSED to happen with GNU licensed stuff?