JTS was founded in 1886 by people who were observant and at one time in the 1920’s or 1930’s a merger with YU considered. Their movement didn’t seperate itsself from Yiddishkeit until the 1950’s when they issued a conservative psak allowing driving to their services on Shabbos, but forbiddiong any other use of cars on Shabbos. It’s been a downhill spiral from then with decreasing observance and acceptance of lifetyles contrary to halacha. The JTS faculty had many frum rabbonim at Heschel’s time.
If he were alive today he wouldn’t recognize what JTS, the Schecter schools or the conservative movement has become.