Near as I can tell the POWER9 chips (at least the IBM ones) only have backwards compatibility through POWER7 from 2010. The last PPC chip Apple used, the G5, implemented the POWER4 ISA, so that's pretty far back.

Haiku should be portable to it, but that's a lot of computer for an OS that is not daily-driver grade.

Haiku will never support legacy BeOS PowerPC apps though, as that makes no sense. PEF is hard to support even today.

It’s a shame, but it’d probably be easier in a VM.

There is also currently no way to even virtualize BeOS for PowerPC. I wish there was. The closest I have seen is a MESS that could boot and show the boot splash screen (this is a 3d Be logo on PowerPC, the one you see on Intel was only ever on Intel.) The dingusppc have BeOS as a possible target for their Mac emulation, but it is not there yet. I'm pretty sure QEMU can't boot BeOS because BeOS requires an old world ROM and it requires a PCI Mac with a 603 or 604 processor - so a 4400, 7300, 8500/8600, 9500/9600 and a few others (including clones) work - but nothing NUBUS and nothing post the first motherboard revision of the 8600/9600. The later x600's have a different motherboard and do not work. Nothing that shipped with a G3 and nothing with new world ROM.

Nope. It really never even supported G3 officially. It will boot on the BeBox natively an a Mac with a boot loader. Without modifying the kernel, I doubt it will boot on much else.