Actual B12 deficiency is very rare in our society, because so many animal products provide ample B12, and our body is actually good at storing it, unlike most of the water-soluble vitamins. Someone who is a big seafood eater can have many months’ worth of B12 stored - perhaps being able to go for a year without any. Also, lots of non-meat products are fortified with B12, like breakfast cereals. It’s hard not to get plenty. And Soylent has a full days’ supply, too.
B12 is not actually produced by animals, it’s produced by bacteria, but it’s concentrated in animals, because we store it so well. Animals like cows have a lot of B12 because they are ruminants - they eat grass, but their enormous gut system contains bacteria which break down the grass, and those bacteria produce the B12 for the cow. Cows have many trillions of bacteria per liter of rumen fluid!
We also get some B12 from our gut bacteria, but not a very big amount. So if you’re eating a strictly vegan diet, with no fortified foods like breakfast cereals, and have a low-fiber diet (bacteria colonize fiber in our gut), for a long time, you may actually get low on B12 - but that’s not very common.
Usually, a B12 deficiency is not a diet problem, but a symptom of other problems - elderly people sometimes have trouble absorbing it from foods, people with pernicious anemia may not absorb enough, certain drugs may accelerate the loss of B12, etc.
So if you are actually experiencing an unlikely B12 shortage, there’s probably something else going on that you should have checked out. It’s not happening because of the Soylent, it’s just coincidence. But it’s more likely that your experience is not from B12 or from the Soylent… many people have a pinched nerve, or have a panic attack. Your experience is fairly typical, and not specific to Soylent - but if you’re worried about them, talking to medical professionals is the thing to do.
If you do actually have sciatica, then it’s true that B12 has been found to help - but it’s not a primary treatment. It’s an ‘also’ kind of add-on. The primary treatments are pain killers for the short term, and physical therapy/exercise for long term (unless your sciatica is caused by a serious physical problem and surgery is appropriate.) It’s important to be properly diagnosed, rather than treating with something like B12 “in the dark.”
For what it’s worth, excess B12 is fairly harmless, and may actually do you good in several other ways, but it’s not going to cure sciatica (even if it may lessen the symptoms for you.)