Night Shift has wreaked havoc on the monitors of my older mac minis. This appears to be another of Apple's famous "obsoleted by software". This is just vile.
3rd party patches are available but all so to fix the damage that Apple has deliberately wrought on older machines by including the software – but not the controls to turn the stupid software off.
Retire the hardware.
Spend 1-3 hours searching out patches and installing them with no guarantee of success and absolutely no benefits except to hack Apple's stupid hardware "feature" off my machine.