No such thing as a soft boot
Between us, here on the Forum:
If I wanted to simulate a restart of a ControlLogix, I would intentionally fault the controller on a watchdog timeout by building a For/Next loop that's longer than the task watchdog. When the controller faulted, the Fault Handler routine would play the role of the Power-Up Handler, resetting the condition that caused the watchdog fault and performing other tasks to get the program ready to run.
Even that mechanism is not the same as a power cycle, because it's not actually running the powerup tests, and it's running the Fault Handler, not the Power-Up Handler.
If your client needs an authoritative letter from Rockwell stating that there is no "soft-boot" function in the ControlLogix, we can set that up with your local office.