Back in the Edo period, Shin was initially used as girl's name but after that, it became more of a unisex name in the Meiji period and then a mainly masculine name in the Taishō period. Based on some research that I've done (gathering names from passenger lists and US census data), Shin was uncommon for girls in the late Edo period and the Meiji & Taishō periods. Most of the girls named Shin had their names written in kana.This goes back to the fact that 2 syllable names were preferred at the time, more so before the Meiji period. Towards the 2nd half of the Meiji period and the Taishō period, those types of names were quickly shunted out in popularity by names ending in *ko.