An old Mac Mini server was ‘going in the skip’ because it had been replaced as an office server. It was handed to me instead, due to my reputation as an IT dumpster diver.
https://everymac.com/systems/apple/mac_mini/specs/mac-mini-core-2-duo-2.66-mid-2010-server-specs.html (server model: 2.66 GHz Core 2 Duo (P8800))
I did MANY upgrades to get it to the latest release, 10.13 High Sierra. A couple of weeks later, Apple announced that they were stopping further updates. I understood at the time this was because this generation of hardware had a 32-bit EFI boot system, despite being a 64-bit processor and that it was only possible to boot Ubuntu by modifying the Ubuntu OSI image. This may have been true at the time. I’ve tried to get to know MacOS but it sometimes feels really sluggish on this hardware and now I’m not seeing the latest features, I decided to consider giving up the struggle.
Today, I tried to boot an Ubuntu 64-bit image from a ‘live’ USB memory stick I’d built on another Ubuntu system. I wanted to see how it would fail. To my surprise, it booted. I had to go into Settings to add a WiFi connections. The only problem I had was that tinny sound came from the Mac Mini’s internal speaker. When I plugged the headphone socket to the TV, rather than being routed by HDMI and the TV’s stereo sound inputs as Mac OS would do, I had no sound. A full install might fix that.
You get (this) Mac to boot from another device by pressing the <Alt> key at startup.
(This didn’t work on a white iMac with a Core 2 Duo processor when I tried months ago but that was probably an earlier version of Ubuntu.)
On The Outside 
The same iMac will boot from the ‘Debian with Raspberry Pi Desktop’ image from https://www.raspberrypi.org/software/raspberry-pi-desktop/ for PCs and Macs. It shows up as an EFI disk. I’ve now installed it to disk. Everything seems to be working apart from the iSight camera and FireWire. It seems they can be extracted from MacOS but I haven’t done that yet.