I was successfully able to install Debian 11 to a 266MHz Beige G3 (Old World) using the 2020-10-06 NETINST image[1]
Because this is an Old World machine, I copied the initrd and kernel from the CD to my System boot volume, and loaded them using BootX 1.2.2. Note that the initrd in the ISO below is much larger than the default setting for initrd size in BootX 1.2.2, which defaults to 8192 kilobytes. You must set it to 20,000,000 or thereabouts, or the first boot with BootX will fail.
Since the old world macs, don't support GRUB, once you've performed a successful install, you still need to copy over the installed kernel/initrd to a place where you can use them to boot via BootX, from the Mac OS side.
[1] https://cdimage.debian.org/cdimage/ports/snapshots/2021-10-06/debian-11.0.0-powerpc-NETINST-1.iso