I've got a system that looks like it's going to be kept around "just in case" (virtualization has already been explored). Long ago I had another such system but when when it needed to be turned on after several years the PSU had died which was unfortunate as compatible replacements were no longer readily available.
What is the best practice to try and minimize degradation of the PSU and other components when a system must be off for very long periods of time? (and there may not be anyone on site who can be trusted to periodically boot and check it)
Shutdown the system and leave it in the standby state, where the PSU and motherboard are still receiving a trickle of power.
Shutdown and flip the hard power switch on the PSU, leaving the system completely without power but still grounded.
Shutdown and unplug the system entirely.