I have read that before, about the military intsruments, and the orchestral. The military at that time for Mr. Sax would have been the bigger market. I guess we will never know about the elusive sopranino in F, but a little mystery is a good thing I think. At least some composers like Ravel wrote for it, and that is a good thing.This might not be new to you, but I was curious and looked it up.
From what I can gather, Adolphe Sax originally intended for there to be two parallel families of sax, military band family in Bb/Eb and orchestral in C/F. Apparently he had the idea of 7 in each group, from sopranino to contra-bass. The stuff I find online suggests that Sax proposed the family but never made most of the orchestral verisons, so it's possible there is either no F sopranino or only prototype versions. Only the C-melody/tenor, F mezzo-soprano/orchestral alto, and C soprano saxes were produced in any numbers.
This article seems to go over it pretty well.
In any case, I don't know if the sopranino is dying out exactly but it's very niche. History may be kind or cruel to the saxophone itself, certainly the 200 years in western music has seen a shift from violins and pianos to saxes to electric guitars to DAWs as the dominant instrument of the age. The shift from electric guitars to DAWs literally happened in the last 40 years. The shift from saxes to guitars basically happened in the 40 years before that.