Recently I have been installing and configuring Kubuntu on several older 27" 5K Retina iMacs (17,1 Late-2015) that are no longer supported with software updates or security patches by Apple. They are beautiful systems and have no issues other than being abandoned by Apple.
These iMacs boot with UEFI and holding the Option key when powering on will offer a choice of boot devices, including the Kubuntu installer USB I flashed from the ISO download.
Actually booting into the Kubuntu installer was a bit of a challenge though because there is a display power management issue with the AMD graphics in these iMacs that requires passing `amdgpu.dpm=0` to the kernel as an option when booting. If you don't do it, you might *eventually* see *something* to show up, but everything will be extremely slow.
Getting that kernel option in place can be a bit tricky if you can't catch the Grub bootloader in time. One way that helped me was to install Ventoy on the USB stick and then copy the Kubuntu ISO to it. Ventoy gave me a "Boot Kubuntu via Grub2" boot option. That gave me a chance to catch Grub with the ESC key and then `e` to edit the options and put `amdgpu.dpm=0` inline with the `quiet` and `splash` options that are were already there.
Proceeding with the install should cause the installer to install grub with that kernel option in place. But if you want to double check or find any painfully slow boot/display issues later on, you can always make sure that option is set in /etc/default/grub and run update-grub to put it back in place.
