Considering every machine needs a particular tweak to get to boot anything other than System 7 or MacOS already, the effort of generally supporting OldWorld PowerMacs is simply too much work. Additionally, the contributors don't have any of these around anymore.
Contibutions are welcome, OldWorld will simply not be supported otherwise.
In general:
You will need a Apple System installed that can run BootX. Outlined here: https://wiki.linuxquestions.org/wiki/Linux_PPC_on_oldworld_macs
If you have enough RAM available you can just boot the kernel and initramfs from the CD via BootX and try your luck.
BootX can be downloaded here: https://www.penguinppc.org/historical/benh/
Alternatives to BootX: https://www.penguinppc.org/bootloaders/