I’ve been running El Capitan, the last officially supported MacOS version on my 2008 Mac Pro for several years. A couple of years back I dipped a toe into using OpenCore Legacy Patcher‘s boot picker to dual boot Proxmox from a separate drive, which has worked well.
It hasn’t been without some GPU related stress though, as at some point between changing monitors (I no longer have the previous monitor to go back to), it was outputting a resolution that wasn’t supported by my new 4k monitor and I couldn’t get a boot screen to appear to be able to select between MacOS or Proxmox, so it was stuck booting Proxmox by default and I ran it headless. This wasn’t too much of an issue as I mainly use the machine for running dev/test VMs and use it as my locally hosted GitLab server.
I did manage to get the resolution issue fixed by remote VNC’ing into the machine while there was no monitor output and change the resolution. Note to self, make sure you have ssh and VNC installed when working with temperamental old hardware.
The only other issue that’s been bothering me for a while is the discontinued support for Chrome updates on old MacOS versions:

10.13 is High Sierra. If I’m going to do something drastic and go for a major unsupported upgrade, I might as well go for the latest version I can install with it still be usable. Opinions vary for the Mac Pro 3,1 but I decided to go with 11, Big Sur:

Fingers crossed. I do have 2 bootable drives with MacOS in this Mac, so if the OpenCore boot picker doesn’t get hosed, I should be at least able to boot El Cap from a second drive and recover from there if needed:
- ssd 1: El Cap: this is the one I’ll reinstall with OpenCore Legacy Patcher and Big Sur
- ssd 2: Proxmox
- hdd 1: El Cap
Hopefully the next update I’ll be able to comment on how successful the install went …