There is also Adélie Linux, which has a big-endian ppc64 and ppc32 flavor, though they have smaller repositories and are purely musl libc focused. Void's primary ppc64 focus is actually little endian and modern hardware (POWER8 and newer; the big endian port also works on modern hardware, in addition to old hardware), these old macs are mostly a side effort and side effect (they *are* pretty much retrocomputing at this point, and I fully expect the community to pitch in with their own fixes and improvements if they want things to work *properly*, I test it on a few machines but I largely deal with POWER9 and similar), we also have bigger repositories and both glibc and musl support.
My main issue with Debian is that they're pretty much keeping up the same old port they've always had. That means, 30 years old inferior ABI on ppc64 (meanwhile, Void, Adélie, FreeBSD use the same modern ABI that was created for new little endian machines even on big endian flavors - which has faster library calls, no function descriptors, simpler to program for etc.), yaboot and its poorly hacked together suite of scripts that can't deal with removable media or modern SSDs (instead of just using GRUB), an assortment of problems across various software that nobody seems to care about, etc. - meanwhile we aim to have the same modern system available everywhere, without getting stuck.