People hear about binning and assume that every product decision has to be related to binning, but usually it's not, it's just market segmentation. AMD had over 80% of Zen2 chiplets coming off the line with 8 fully-functional cores, and clock bins are generally selected such that most units will pass, by design. And that's at launch, on a new node, in 2019. Numbers have only gotten better over the last 3 years.
AMD already has a bin for iGPUs with a defect - it's 5600G/5600U/5600H/etc. And they have 5300G below that allowing even more defects. There's very very few APUs coming off the line with tons of GPU defects but 6 workable cores, or a defective PCIe controller but only a defect in the iGPU part and not the rest, etc.
The problem is that AMD has tons of supply of high-binned parts but the lowest demand for those parts. And they have the highest demand for low-binned parts but the lowest supply of those parts. How do you mesh those two curves? Disable cores on a high-binned part and sell it as a lower SKU. That's why those "black edition with unlockable cores" existed - those unlockable cores were locked off for market segmentation. Nowadays they just don't let you turn it back on.
(Which isn't to say that none of the 5600X/etc are the result of a dead core/etc - but a lot of them aren't, probably most of them aren't, given the likely >90% yields for 8-core at this point. And you pick your 5600X bin such that most 5800X failures can be sold as 5600X, meaning there's very little that falls through the cracks without being just utterly broken. True binning-generated "we have this pile of chips, let's do something with it" style SKUs tend to have extremely limited availability as a result, it's shit like the Ryzen 4700S or the Ryzen 3100, or the NVIDIA 1650 KO.)
Anyway, it's not a coincidence this is coming a few months after the Alder Lake launch. This is market-driven, Alder Lake is not only faster but in many cases it's cheaper as well. AMD coasted a little bit while motherboard supply firmed up for Intel, but they finally have to respond. I'm sure they're selling lots of Milan but consumer marketshare matters too, and AMD is losing steam there with the price increases, with Intel back on top on performance, and with Intel undercutting their pricing heavily.