Fermentable fibers aren't just important, they're one of the key determinants of how healthy your diet is. Mood, memory & cognitive function, immune function, inflammation and leanness are all strongly influenced by your microbiome. Not only does a high fiber diet promote an optimal microbiome, the byproducts of fiber fermentation are actually the mechanism for most of the benefits.
Soylent also fails due to a lack of health-promoting phytonutrients. Between this and the lack of fiber, it is really a poor choice. You really need to get one solid, plant based meal a day for proper health. A better on-the-go alternative to soylent is a mixture of nuts and high phytonutrient dried fruit like blueberries/cherries (without added sugar). Tempeh also works well on the go and it is extremely healthy.
Do you have any sources for how important fiber is? I usually eat keto, and when I tried adding fiber to my diet I got stomach cramps, felt like I was hungry all the time (which never happens), and grew extremely irritable. Over the next few days I had two or three massive, green bowel movements, which was also unusual since I generally only go twice a week. Previous times I've tried taking some psyllium husk and even that seemed to lead to hunger pangs and bowel movements.
I'm always interested in improving my health, so I'll definitely try it again, perhaps more scientifically this time. That said, I don't want to struggle through the annoying symptoms if it turns out that fiber doesn't do anything.
I'm not surprised you suffered gastro-intestinal upset when you added fiber to a keto diet. Your gut biome was adapted to a low fiber, high fat diet. Rapid changes in dietary composition will frequently cause this issue. One solution to this problem is to consume your fiber along with a probiotic supplement, or preferably a live culture fermented food such as unpasteurized sauerkraut/kimchi/greek yogurt. Beans (particularly lentils) are the most nutritious fiber source, but if you want to stick with the keto diet, you should use inulin instead of psyllium husk, as there is more research demonstrating its efficacy. You may still encounter some GI upset even with a probiotic, but it should decrease as your gut biota adapts over the course of a week or two.
For references, there are plenty. Specifically want to look for material related to short chain fatty acids, particularly butyrate, and their role in modulating the immune system, and as histone deacetylase inhibitors. Here are a few reviews to get you started:
Additionally, I should note that blue zone diets are all high in fermentable carbohydrates, and increased concentrations of short chain fatty acids have been observed in the stool of centenarian populations:
Soylent also fails due to a lack of health-promoting phytonutrients. Between this and the lack of fiber, it is really a poor choice. You really need to get one solid, plant based meal a day for proper health. A better on-the-go alternative to soylent is a mixture of nuts and high phytonutrient dried fruit like blueberries/cherries (without added sugar). Tempeh also works well on the go and it is extremely healthy.