I’ve been on a plant-based (“vegan") diet for 10 years and haven’t had any deficiencies. I take a B12 supplement a few times per month and some of the food I buy is fortified as well.
One somewhat random question I have is: How do herbivorous animals (cows, pigs, etc.) get their B12? I think you mentioned that it’s produced by bacteria in their gut, but I’ve heard the bacteria that’s used usually comes from the soil (e.g. a non-animal origin). I’ve also heard that some farm animals are given B12 supplements.
I know humans aren’t capable of producing B12 themselves, but are other species?