Я пытаюсь найти работающую веб-страницу, с которой я могу загрузить файл ISO для последней версии (20.04), который я могу установить на платформу PowerPC (ImacG5).
As far as I know ubuntu doesn't support PowerPC architecture in the newer releases anymore(To be precise , from Trusty Tahr or 14.04 onwards).However you can download the older versions like 14.04 but that's not maintained in the ubuntu official download repository anymore.So you have to use the Wayback Machine.
Fortunately I was able to find a working download link for you that was saved in Aug 2015 (If fact it's Lubuntu which has a lighter desktop environment and is more conservative in memory usage) :
But there is also an alternative : You can use Debian for PowerPC if you want to have a newer kernel and more up-to-date and secure packages in your pocket because the PowerPC architecture is supported officially by the Debian group.
But note that despite of the advantages that a newer OS has , it can be quite difficult to run a new OS smoothly in your iMac because the hardware is rather old. Looks like you have 512 MB of 533 MHz PC2-4200 DDR2 SDRAM Expandable to 2.5 GB in your iMac G5 according to Wikipedia.See Debian hardware requirements.
For more information see this guide.
Hope it helps.
EDIT: Ubuntu has PPC port as well but it's unofficial and is maintained by the community.It's very difficult to find the download links for it.I don't know why.But at least I found a Ubuntu Server 20.04 for POWER architecture but not PowerPC and I don't think you can run it in your iMac.