Anecdotically, 64GB works extremely reliably on 8GB cards after unlock.

It is much more of "let's sell cards to crypto people, but nerf them, to not harm our enterprise margins", not "we've got a ton of subpar hardware, let's to something with it".

I have a grey market a100 that had hbm errors but the card used some of its spare rows to remap. I believe they introduced this with ampere, and I was a little annoyed that my “new” card had these but I got it so cheap I didn’t really care. It was 7k usd (vs 9k and 10k) and I remember being unsure if it was worth it… None of them needed to remap any more in they years I’ve had them but I think I got that one so cheap because of it. The most expensive one came from the best rated seller and appears to be an engineering sample but has passed every test I’ve thrown at it, the only thing “wrong” with it is that I can set the power limit to whatever I want? This was back when b100 was about to be formally announced and before the pricing so eBay sellers were trying to unload them.

Binned for compute failures, not memory failures.

That's why this works. The yield on those HBM stacks is excellent. The binned cards had failures in the logic die, and hyperscalers want uniformity in their clusters. That's why the mining market took the downbinned cards.