AVR-LIBC hasn't seen any activity in a subjective age. Bug Reports and Patches get posted to the official AVR-LIBC repository and no action gets done. Nothing. No rejections, no consideration, nothing.
I wanted xmega3 devices to be in AVR-LIBC, there is a patch for that. There are other patches too, like a delay routine for attiny.
SO, I have converted the entire SVN history from the official AVR-LIBC to git. I have applied 6 long outstanding patches, the oldest dating from July 2017. It adds support for 26 processors, fixes definitions for ATmega324PA and other issues. I have called the fork AVR-LIBC3 because I don't want any confusion with AVR-LIBC which is the official version and its last release was V2, unmaintained as it is. If anyone has any bugs they want fixed in AVR-LIBC or wants to benefit from having a maintained source tree of AVR-LIBC, feel free to clone and push merge requests. There are a few Patches I haven't applied, because they don't apply cleanly. like new/delete support for C++. There are numerous issues in the bug tracker, some with patches. I haven't even begun to look at those.
Bringing AVR-LIBC up to include all the work that people have done, but the maintainers have neglected, is a big task.
Feel under no obligation to use this, but if you want modern tools, with support for modern versions of GCC, actively maintaining AVR-LIBC is going to be necessary.
Source is here: https://github.com/stevenj/avr-libc3