I mean the ATX standard has this signal built in, so Apple could just copy it:
I don't think that quite works for the purpose. What you'd want is a second signal that goes low as soon as possible after loss of AC power.
My reading here is that PWR_OK going low is an indication that the PSU has stopped providing good power, and the CPU must shut down immediately, or it might miscompute something due to low voltage. At this point you absolutely don't want to do any last-minute writing, you'd be risking corruption.
What you need here is an early warning signal that you can react to while the PSU is still coasting on the internal capacitors.
I would has a guess that 16ms is the physical limit for most consumer hardware (and maybe commercial computing) to detect mains loss.
Of course there is industrial hardware that can detect quicker than this but it would add a LOT of cost for arguably little gain, or something that could be solved in another manner.
Doubtful. 16ms is an awfully long time these days. There's no reason why you couldn't detect power loss much sooner, given a good input signal. The concept also gets used quite often, in the form of SSRs with zero crossing detection. Those are used for dimmers.
The reason is likely related to the awful waveforms produced by some UPSes and inverters:
https://www.christidis.info/images/blog/scope_20.png
Unlike a nice sine wave, those spend a good while hovering near zero volts, so the PSU has to be able to tolerate that. Detecting loss of power sooner in this case isn't a question of cost, it's a question of that you don't have a good signal to do the detection on to start with.
That wave chart was atrocious. I wonder if the extra load on the DC-side caps leads to them having lower life expectancy than the ones in a PSU attached to a proper power grid?