Once year, things freeze up, mouse etc, and hard force quit with button on imac.

But first time this annoying window came up asking if I would like to shut down. Nothing worked, corded mouse, even white Mac bluetooth mouse, no keyboard, so had to unplug the computer to reboot which had not done in 10 years.

But why could I not just use the computers on off button, is this a new nuisance?

The only thing that comes to mind is that you didn't hold the off button down long enough.

I don't remember when things changed, but for some time now, I've had to hold that button down for almost 5 seconds before my MBP shuts down.

I've got no idea why yours didn't shut down when you responded to the prompt.

I'll try remember that, not sure if I did 5 or 3 seconds... but I should not test that, because, its not good for computer to do that... force quitting everything without ejecting hard drives.

But still works?

It worked for me in Catalina not too long ago.

The power button shutdown is a hardware feature unaffected by macOS updates. Theoretically it still works on by M1 and Intel Macs. On M1 Macs, when and how long the power button is pressed is an essential control feature. Supposedly it still works if you hold the power button depressed long enough. The situation you describe implies the entire I/O system was frozen and it is conceivable the power button as well, but it might take a full thirty seconds or longer depending on what other signals are blocked or slowed.

Interesting Joe, non M1, 5 year old 27" imac.

Yeah, did not wait long, just normal punch in..

So will make a note to wait longer in future.... Just got that window, asking if I wanted to really do, what I wanted to do...!