I heard anecdotally that the early JTS president Solomon Schechter was not, but I have been unable to find a citation either way on this claim.
Schechter was allegedly one of the most accomplished Jewish academic scholars and had enormous influence on the development of American Judaism, for better or worse. Consequently, knowing whether he was shomer shabbos has large implications for how we view JTS and Conservative Judaism as sociological movements at the beginning of the 20th century. Further, the answer to this question has possible implications on how we view contemporary halachic innovation movements such as YCT.