For context, I should mention how science discovered and defined the essential vitamins and minerals. These are things that were found to cure obvious and damaging diseases, like scurvy or pellagra, that resulted from really bad diets. If enough people had a nasty disease, and a vitamin or mineral supplement cured that disease, then it was classified as essential.
I suspect that there are many other semi-essential things whose absence could cause small problems, or large problems over the long term. Soylent starts with simple stuff and adds the known essentials. If you tried to live on it for years, you would probably develop a new and interesting nutrient deficiency disease.
However, if you lived mainly on Soylent and also ate a salad a day along with it, you would probably be fine, especially if you added in fresh fruits and occasional seafood.
I would also recommend eating some kind of real food at the same time that you drink the soylent, because many nutrients that are good in food are much less useful in supplement form. It seems that the body has trouble digesting some things properly when they are not in a form that resembles a natural food.