Note: This is an excerpt from a post I posted on the Anarchoprimitivism subreddit with some important corrections. You can find the original thread here.
For veganism to not negatively impact your health due to poverty of important nutrients such as certain vitamins (K2, B12, protein, fatty acids), you have to rely on plant foods that 1) aren't really possible to amass in quantity solely from foraging and 2) that require additional processing in order to obtain the necessary nutrients. I'll take each point in detail.
1) Most plant foods (fruit, tubers, roots, various aerial parts like stalks, shoots, and leaves) are quite poor in protein and fatty acids, and, depending on the climate, only seasonally available. Plant foods that are relatively richer in these nutrients, such as the seeds (which include nuts, beans, legumes, etc.), are either impractical to gather in sufficient quantities to adequately satisfy nutritional requirements in humans or are otherwise impractical to process--i.e., you may have tons of acorns or walnuts, but it is extremely labor-intensive to extract the edible portions of the food. In the case of acorns, the tannins need to be leeched from the acorns first before the acorns are pounded into flour. In the case of walnuts, one Native American method was to crush all the nuts and boil everything until the meat floated and the shells sank to the bottom of the pot. This would take hours (gather, crush in batches, boil in batches, strain, repeat), as opposed to just snatching a few blackberries along the path. As a supplement to a diet that includes animal foods, it may be reasonable to boil walnuts once in a while to add variety. As a staple, though, you'd be rather tied to this method of preparation for lack of alternative sources of nutrition. What's more, many plant foods (including walnuts) contain "anti-nutrients" such as phytic and oxalic acid that interfere with the absorption of nutrients. These can be reduced by extended cooking and/or soaking methods, or by germination. All require extra time and effort. Animal foods generally do not contain anti-nutrients.
In general, it is less efficient to derive certain nutrients from plants than it is from other foods, including animal foods. The point about anti-nutrients notwithstanding, many nutrients touted in plant foods actually don't exist in an immediately bioavailable form, or are less efficiently absorbed. For example, most people associate carrots with providing loads of Vitamin A in the form of beta-carotene. However, getting Vitamin A in the form of beta-carotene turns out to be 1/12th as efficient as getting it in the form of retinol from animal sources. By weight, beef liver has about three times as much retinol as carrots have of beta-carotene, which means that it is about thirty-six times less efficient to derive Vitamin A from eating just carrots than it is from eating just beef liver. Another example would be the omega-3 fatty acid alpha linolenic acid (ALA) found in abundance in flax seeds. ALA is one of three omega-3 essential fatty acids, the other two being the more important eicosapentaenoic acid (EPA) and docosohexaenoic acid (DHA), the most crucial of the three. Although humans can synthesize EPA and DHA from ALA such as found in flax seeds, the efficiency of either conversion is only at a few percent. In animal foods like pastured animals and wild oily fish, DHA and EPA are readily available for absorption and doesn't require any conversion. This makes sense because components from animals are more easily recognized and easily repurposed in our own animal bodies since they are more or less going to be applied to similar purposes from one animal to the next. The calcium in cow's milk or in bones can be very easily repurposed for your own bones--the calcium in broccoli, not as much (again, partly due to anti-nutrients). Add to this extra step in metabolizing nutrients from plant foods the fact that digestion of plant foods is less efficient than animal foods, and it starts to become apparent that relying on plants to supply the majority of one's nutritional needs is not a choice strategy from a pure survival viewpoint. For example, it is relatively common to find undigested plant foods in the stool of someone with weak digestion, but it's relatively unheard of to find undigested muscle fibers from a chicken in the same person's stool.
Other staples that vegans typically depend on, like legumes, aren't available in sufficient quantities in the wild to even sustain a small population of humans. Thus, these plants need to be domesticated in order for the vegan strategy to possibly work. In modern veganism, for example, soy is an important source of protein and there has yet to be a suitable substitute for it. This leads me to the second point.
2) Because of the high amounts of anti-nutrients in some raw plant foods, particularly those valued by vegans as substitutes for meat-based protein, many agricultural cultures have learned to ferment or otherwise process certain foods in order to reduce or eliminate anti-nutrients and make other nutrients available. Taking the example of soy again, East Asian cultures have a long tradition of fermenting the bean into soy sauce, natto, miso, and tempeh. Of these four fermented soy products, only natto provides Vitamin K2, a nutrient critical for bone health and proper blood coagulation that is generally not found outside of animal and fermented plant foods. While I can't speculate as to whether these cultures understood the benefits of fermentation in terms of nutrition versus just making things taste better, the fact is that fermentation effectively reduces the phyto-estrogens in soy, which can promote several types of cancers. Fermentation of certain plant foods (but not soy) also makes Vitamin B12, a critical nutrient generally only found in animal foods, available. Fermentation, however, requires well-controlled conditions in order to work properly, which implies sedentarism and a degree of technical sophistication. While some hunter-gatherers do consume fermented foods, these foods aren't generally the product of sophisticated techniques, but rather of just letting foods (like meat) spoil or finding food already in the process of fermentation.
Another major processing method that basically can only exist under sedentary agricultural conditions include pretty much all grains, as these foods a) can't be gathered in sufficient quantities in the wild to be worthwhile and thus need to be grown in large quantities and cared for accordingly b) require threshing, hulling, winnowing, and milling before they are edible, which are only practically achievable through various specialized tools and machines c) as grain harvests are seasonal, require suitable storage conditions that will keep the grains dry and free from pests, again implying sedentarism.
Probably the closest analogue to modern veganism is found in the East Asian Buddhist vegetarian traditions, which, because dairy foods were not consumed in these cultures, relied more heavily on the versatility of the soy bean, leading to the innovation of many unique and iconic vegan foods such as the aforementioned soy products as well as tofu. An imitation meat made from wheat gluten also exists and is part of traditional East Asian Buddhists' diets. It can be said that modern veganism owes much to East Asian Buddhism, and, as with modern veganism, East Asian Buddhism owes much to vast fields of soy beans, none of which the monks cared for themselves. East Asian Buddhist monks were not traditionally very physically robust or even healthy. They generally consumed one meal a day. They did not have to be healthy in the same sense that people who, say, farmed for a living needed to be healthy. Since monks didn't devote any time to growing or cooking their own food (they needed that time to study sutras and perform religious functions), East Asian Buddhism doesn't really qualify as a sustainable culture. They feed off of the labor of others, the same as any other organized religion. I've heard that Buddhists were originally prohibited from growing their own food, since this would entail tilling the soil, which results in the death of soil-dwelling animals. Generally, pre-modern monks got fed strictly off of the donations of faithful villagers. Thus, the case of East Asian Buddhism doesn't really apply as an example of a fully vegan culture, since monks basically did nothing directly to feed themselves, and the farming culture that supported the monks certainly was not vegan.
In conclusion, an exclusively plant-based diet (including bacteria and fungi), because of the difficulty inherent in procuring the nutrition that humans require to remain reasonably healthy, absolutely requires advanced agricultural knowledge, capability, and labor. Again, to my knowledge there are no pre-modern vegan cultures, most likely because such a diet is inefficient, unhealthy, and impractical.