In context of soylent, I don’t - and am not sure we can.
In context of established dietary research, though, I feel it’s important to bear in mind. Foods that are intrinsically high in fiber tend to be more healthful, and foods intrinsically low on fiber tend to be less so - which means the research findings favoring fiber-rich diets may well be favoring healthier foods, as opposed to the fiber, itself. Fiber may signal healthy foods, rather than being healthful, from a nutritional standpoint.
Soylent is designed to be nutritionally healthful regardless the fiber content, so the fiber content is not a signal of healthfulness.
I’m not saying any of this with certainty; I’m just saying the case for adding raw fiber to Soylent is far, far weaker than general nutritional advice on fiber would imply.
It would be interesting to see some BTT studies on the new, lower - fiber formula, to ensure things really are “moving along.”