Simulating a dirty shutdown seems no longer possible on macOS 14.4 and later

In macOS 14.0 and earlier, the shutdown command had a -u option for handling UPS-initiated emergency shutdowns. From the man page of shutdown on an old Mac: > -u The system is halted up until the point of removing system power, > but waits before removing power for 5 minutes so that an external > UPS (uninterruptible power supply) can forcibly remove power. This > simulates a dirty shutdown to permit a later automatic power on. OS > X uses this mode automatically with supported UPSs in emergency > shutdowns. On macOS 14.4 and later, this option was removed. In fact, Apple publishes the source code of the shutdown utility on GitHub , and commit 49dcc07 ("14.4") effectively removes all code that handles the -u option. Sure enough, by shutting down a machine running macOS 14.4 or later with command shutdown -uh now, the following message is printed to the console:

shutdown: -u has no effect

Anecdotally, I can say that I simulated a power loss on my UPS until my Mac mini (running macOS 15.1) automatically shut down. I waited for the UPS to completely run out of battery and, when I plugged it back in, the Mac mini did *not* restart, even if the "Start up automatically after a power failure" option is enabled in Settings. If my experience is confirmed, it means that Apple did not just remove the option to *manually* simulate a dirty shutdown, but they removed the feature entirely, even in the case of actual UPS-initiated shutdowns. How can we trigger a dirty shutdown on macOS 14.4 or later? As of now, it looks like there is no way of automatically restarting a desktop Mac after an emergency shutdown.