So what is the latest OS you can run on that hardware?
Well, think of it like PPC is not the limiting factor ... everything is in the flow!
Let's say, it's the idea of PPC ...
How getting to PPC-Macs and getting here is another story - like with anyone else.
I've learned a lot about to honor, use and improve old and newer computers and other electronic-stuff
instead of only looking for the latest and greatest.
It's like taking the right tools for the right job - and sometimes just fun is the job.
This is my fleet:
os9: on Clamshell, iBookG3, TiBook, iMacG3, Cube-G4, PowerMacsG4 (got my first Mac in '09 and when I started to get engaged with os9 3y ago it was like catching-up missed history ...)
Tiger/Classic and Leopard: on G4/G5 Macs
Lion: on white c2duo intel-iMacs
Mojave: on early-2008 MacBookPro's and above (made possible by @dosdude1 's great patches.)
And finally ElCapitan on my fastest Mac: a mid-2012 nr 4x2,4GHz 15"MBP (and I don't dare to upgrade that machine to anything else ...)
Unfortunately PPCs don't cope with VPN/RDP-sessions to my Office-WinServer-Network, so an early-intel became my daily driver: because it resembles the late PowerBooks and because of it's superb display.
The main goal and fun is, to keep all machines connected: file-sharing/SMB/AFP/FTP; screen-sharing/VNC/RDP; common Cloud-space: webDAV; IMAP; etc. )
It all fits well into a Mac/Win/Linux-environment. (And certainly BSD too ... - I write this because I'm dedicated to BSD, though I think I'm to stupid to understand)
That's it.