I don't actually know whether they removed features outside of detailed reporting... which is the insidious thing about removing detailed reporting.
I think you could run different tests for different things. I don't know if Apple Diagnostics just runs them all, but it's a single progressbar.
Your G5 certainly has a problem there! I'm not too surprised AHT didn't find it, the scope of software like that is limited. I wonder if it's something like a weak short circuit - bad enough to draw so much current the PSU ends up failing, but enough ohms that it doesn't cause enough voltage droop to prevent the computer from working at all.
I don't know what triggered it, as I only saw it die once with my own eyes, when entering sleep. But it failed three times in the same way. One of the components on the PSU's PCB exploded with a loud bang and a flash of light. It was quite spectacular, if nothing else.
I originally bought it non-working (sold for parts) and after some time trying to troubleshoot it I narrowed it down to the PSU. I disassembled it (which is a lengthy process on the G5's, the PSU can only be extracted after removing the CPU heatsinks and daughtercards) and found a broken component on the PCB by visual inspection. I remember that the name was unreadable (see pic) and took me a while until I found the name online (not many G5 PSU teardowns on the internet, iFixit wasn't around then). All I knew is that it was MOSFET-looking (but not a MOSFET, as someone on the internet pointed out at the time, it had too many legs for that).
After I found the name (
TOP249Y, in case you're interested), I bought another one online. I still had no clue what it was, the datasheet/diagarm didn't mean much to me at the time (I was 15), but I soldered the new one right in. It was also my first time soldering anything, so I remember being quite excited when I got it to boot. I ran the AHT, to check that everything was working and expected, and set up the OS. About a week later it exploded again, so I assumed I had botched the soldering job and resigned to buying a whole new power supply. This is the only time I saw it explode with my own eyes, with the whole flash of light + loud bang + strong burnt smell.
The new PSU came, I disassembled it AGAIN, installed the new one, re-assembled, and got it up to speed again. This lasted the longest, a whole two weeks before it exploded again (I believe also while going into sleep mode), but I didn't got to see it. After that, I had disassembled it way too many times, and didn't want to keep breaking PSUs as it was clear that something else was going on (and the G5 PSUs were a limited resource, Apple had stopped making them long ago).
I still have it, but haven't looked into fixing it ever since.
Here's a pic from the time: