I managed to get OpenBSD running on a PowerBook. It was not straightforward, but eventually I have it working. However, none of the modern browsers build (ironically, TFF may be still better than what OpenBSD can offer), XFCE is broken due to careless dependency specifications which pull in unnecessary stuff, etc. Ah, there is no support for FireWire.
So while it can be usable, it is rather painful.
I was thinking to try NetBSD, but cannot get it installed. I did not have time to spend hours on that yet, but a few initial attempts failed with a variety of errors both on G5 and G4.
FreeBSD 14 is just as broken on multicore G5s, and on G4, while the OS itself installs and runs fine, I got stuck at broken Perl:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=276861
Obviously, with a broken Perl nothing can be built, and FreeBSD offers nothing pre-built, so it is useless atm.