You know what? I’m glad I asked this question. I wonder why the phrase is so widely used in reviews. Maybe it sounds cool.
It's not being used b/c it's cool. It was speculated that Apple was actually doing binning on the M1 chips when one of the GPU cores was bad, resulting in both 7-core and 8-core versions (
https://www.anandtech.com/show/1623...-2020-macbook-air-13inch-macbook-pro-mac-mini ). So that language simply carried over when distinguishing the low-spec vs. normal-spec M1 Pro. As to whether they're actually doing binning there or not, we don't know.
I would guess there are at least *some* defects, so there is probably some true binning, in combination with deliberate core disablement to meet sales needs.
I don't believe the 8 core chips are a result of binning. There are so many other parts to these chips that it would be vanishingly improbably that two GPUs (and only two GPUs) wouldn't work after production.
Since the OP was referring to 8-core vs. 10-core in the M1 Pro, that's CPU core count reduction, not GPU core count reduction (separately, there is also GPU reduction, from 16-core to 14-core).
As to the probabilities, it could be that Apple's analysis indicated a reasonable probability of one CPU core being defective, with a small probability of two. Thus, to simplify things, Apple decided to only offer a single 8-core binned model, which covers both possibilities, but where most of those 8-core binned chips actually have only one defective CPU core rather than two. I.e., it's not required that two CPU cores failed for it to be a legit binned chip.
In addition, true binning would result in limited supplies of Apple's most popular base model MacBook Pro. And we never see that happen. Combined with a mature N5 process, it doesn't make sense for binning to occur except to satisfy marketing requirements.
If they need more of the low-spec chips than they actually see due to defects, they could always take non-binned chips and disable the cores to supplement the binned chips. So the supply issue doesn't mean true binning isn't actually going on.