Non-vegans love to point at B12 to discredit a plant-based diet as unnatural, while ignoring the fact that most supplements exist for and are consumed by omnivores. Milk, flour, bread, cereal, juice, table salt are all commonly fortified or enriched with various nutrients of modern concern including vitamin D, C, iodine, B12, riboflavin, thiamin, niacin, folic acid, calcium and iron. And why would an non-vegan ever need or want to take a protein supplement?!
In the early 1900s, goiter (a disease of the thyroid gland) was relatively common in areas where iodine was deficient in the soil. In 1924, some salt makers added iodine to their product, which helped reduce the number of new cases of goiter dramatically within a short time.
Milk was first fortified with vitamin D in 1933 to ensure that a sufficient amount of calcium would be absorbed. A vitamin D deficiency can lead to rickets in children and osteoporosis in adults.
Today you'll find calcium-fortified orange juice, phytosterol-fortified margarine and vitamin and mineral fortified breakfast cereals in your local grocery store.
Also note that factory farmed animals, which make up 95 - 99% of animals in agriculture https://www.aspca.org/animal-cruelty/farm-animal-welfare
have such poor living and feeding environments they must be given supplements such as cobalt for ruminants and B-complex (including cyanocobalamin) for pigs and birds:
In contrast, pigs and poultry require an external source of supplementation for all B vitamins, as the microbial fermentation in their large intestine does not cover their needs under modern production conditions.*
In pig and poultry diets, B vitamins are usually supplied through the medium of a vitamin and trace mineral premix that provides all vitamins and minerals -- that is, it ignores contributions from natural ingredients.
So a non-vegan that consumes bread, cereal, milk, salt and juice are taking in supplements without concern and commonly championed for good health. Meat products have usable levels of B12 in part thanks to supplements given to the animal.
But ultimately, supplementing, fortification and enrichment are practical solutions to common modern health concerns which we all gain benefit from whether we realise it or not!