Multiple phonetic and phonological studies document natural languages that maintain a phonemic distinction between s and sh sounds.
The claim that natural languages exist with distinct s and sh phonemes is a standard linguistic fact supported by numerous phonetic and phonological studies in the literature (e.g., English, Japanese, Slavic languages, and sibilant harmony systems). Papers [3], [5], [7], and [9] all explicitly reference or rely on the phonemic distinctness of [s] and [ʃ] ('sh') in natural languages. There is no evidence in the provided sources refuting this well-established linguistic universal.