I keep seeing this claim in these HN threads and across the internet, do you have a reliable source for this? Google brings up nothing but vegan conspiracy sites.

Found this interesting article from someone that seems to know cows: https://praisetheruminant.com/ruminations/is-it-true-that-co...

Yeah, this article makes more sense. My grandfather put out the blue salt blocks every year so the cows would get enough cobalt (I remember asking teachers why salt blocks were blue until one knew why). That's why this all seems like nonsense, none of the farmers I know ever gave their cows B12 supplements or have even heard of it. Now their VET might have given B12 shots to the odd calf like that article states, that I would believe.

If this is true, what was the dietary source of B12 in pre-modern times?

Trust me, it's always a rabbit hole; with claims like "humans used to be entirely, or mostly, vegan and got our vitamin b12 from eating the dirt on root vegetables" or "if we stopped washing our fruits and vegetables we would get enough from the bacteria on the produce"

Things can be complicated I'm not sure that is a reason to think these theories are incorrect. I am tempted to believe microbiome and eating more slightly rotten/off food than we currently do also helped but it's not as if humans from antiquity were all living into their 90s.

Well, except there is no evidence at all that humans were ever anything other than omnivores and large segments of humanity were know to be hunter gatherer societies from the archaeological record. A few societies have become ovo-lacto vegetarians and some modern humans have effectively become vegan.