If the energy efficiency of things like Face ID was indeed so far so bad that you need a more efficient M3 Ultra, how come Face ID was integrated into smartphones years ago, apparently without significant negative impact on battery life?

FaceID was just one example they gave (which is probably faster and more energy efficient now).

Image recognition, OCR, AR and more are applications of the NPU that didn’t exist at all on older iPhones because they have would be too intensive for the chips and batteries.

That's false. Face ID is in fact a complex form of image recognition, so image recognition was definitely possible on older NPUs. OCR is the simplest form of image recognition (OCR was literally the first application of LeCun's CNN), so this was definitely possible as well. "AR" is an extremely vague term. If you refer to Snapchat style video overlays, those have been possible for a long time as well.

You seem to be arguing with a strawman here -- who said you need an M3 Ultra for energy efficient Face ID?

"stouset" implied that those are merely possible but not energy efficient on the older mobile hardware.

The original question was asking what features have taken advantage of a NPU. Face ID was introduced with Apple's first "Neural Engine" CPU, the A11 Bionic.

You're confusing this with what features/enhancements new generations of NPUs bring, which nobody else was talking about. Everyone else in the conversation is comparing pre- and post-NPU.

The original question was clearly about the NPU of the currently discussed M3 Ultra, which is twice as large as the previous one. The question is what this one is good for, not what much, much smaller NPUs are good for which have nothing to do with the M3 Ultra topic.