A diet partially or totally devoid of animal products can predispose to the risk of vitamin B12 deficiency. However, this does not mean that a plant-based diet is not natural.
Here there are two reasons:
Plants do not produce vitamin B12, nor is it produced by animals. In nature, it is the bacteria present in the soil or in the water that produce vitamin B12. Therefore, vegetables as they are found in nature (and as humanity has always consumed them when living in a state of nature) contain B12 from bacterial contamination.
We no longer live in a state of nature; therefore, we are not able, nowadays, to take this nutrient from plants contaminated by bacteria, also because the common hygienic practices necessary to remove any contaminant, biological and chemical, must always be respected.
The problem, therefore, is not that "Nature" has badly planned this aspect and therefore eating only plant foods is "unnatural".
The solution is simple: to cultivate the bacteria specifically, exactly as you do for yogurt, for example (with other bacteria strains, of course), and no one finds it strange or unnatural to eat yogurt (obviously soy, in our case).
These bacteria live on a carbohydrate substrate (e.g. molasses). The B12 they produce is taken and placed in a "caramel": that is the supplement. Therefore, it contains vitamin B12 produced exactly as in nature by bacteria.
The second reason why it is not unnatural to take vitamin B12 of bacterial synthesis and instead it is much more unnatural to take that deposited in the meat of animals or dairy products and eggs is that animals take they amount of B12 vitamin from supplements added to their feed.
Farmed animals consume the vast majority of vitamin B12 produced in the world!
So what's the point of not wanting to take a supplement and get vitamin B12 by eating animal products? The same animals are made to take the supplement, and it is only for this reason that vitamin B12 is present in their meat or milk or eggs; otherwise, there would be none!
It is much more logical, natural and healthy to take a bacterial synthesis supplement, that is, the form that would be found in nature. Additionally, farmed animals are not only given vitamin B12 but many other supplements, chemicals and medications.
What's "natural" about this? Not only that farms are the most unnatural thing in the world. Most people who choose not to eat animal products do so out of ethical choice, so we must never forget that the atrocious suffering and death inflicted on billions of animals is certainly not something "natural" and acceptable.
Finally, taking vitamin B12 from animal food such as dairy products and eggs in adequate quantities to meet the required needs means having to take a very high amount, absolutely not in line with the recommendations for a healthy diet, and this would expose you to the risks related to intake of animal fats and proteins found in these foods. The use of supplements of this vitamin is instead an intelligent and safe way to protect yourself from the risk of deficiency, which among other things, is increasingly widespread. The American Institute of Medicine recommends that all people over 50 years, regardless of the type of diet, take vitamin B12 in the form of supplements and fortified foods to ensure optimal absorption.