As in people doing these diets years before soylent, not years on said diet. My grammar might not've been clear enough.
EDIT: Specifically I'm thinking of the hundreds (if not thousands by now) of people on T-Nation/Testosterone Nation who've done the 1-month Velocity diet, which is (or was, I haven't been there in years, but that particular diet has been around for at least 5 years+), a protein shake diet with very little solid food. There are years of people doing it for a month and relating their experiences, highlights and downfalls, and I'm sure there are several more experiences in that world that would provide better data points. However, it has the issue of being mostly anecdotal evidence.
EDIT: Specifically I'm thinking of the hundreds (if not thousands by now) of people on T-Nation/Testosterone Nation who've done the 1-month Velocity diet, which is (or was, I haven't been there in years, but that particular diet has been around for at least 5 years+), a protein shake diet with very little solid food. There are years of people doing it for a month and relating their experiences, highlights and downfalls, and I'm sure there are several more experiences in that world that would provide better data points. However, it has the issue of being mostly anecdotal evidence.