Good news. Due to recent development efforts, arm64 / Raspberry Pi (RPi) builds were fixed in Whonix git tag 15.0.1.2.0-developers-only.
sudo ./whonix_build --target raw --flavor whonix-gateway-rpi --build --arch arm64 --kernel linux-image-arm64 --headers linux-headers-arm64
There might still be minor build issues or unrelated issues due to the recent development efforts. Therefore this is likely to work better when the next stable release of Whonix gets released.
Will edit to add link to recent development efforts.
Note: only the build was fixed. I didn’t try to boot the image let alone try it on real hardware. You could help the development if you could create instructions how to boot that image using virtualization such as libvirt configuration files and/or qemu command line to boot the image from a amd64 host.
It would be good if we had a Debian based CI (continuous integration) server with full support to use mount etc. Then Whonix build script could continue to build RPi builds on that server to make sure that new changes don’t break again RPi builds.