I think you have asked two separate questions on topics close to each other. The OPTO question is fairly new, and I believe it is subject to agreements with the TWU. (I could be wrong.)
As I recall, in the late 1940's both the IRT and the BMT operated short trains off hours. That's the origin of the old enamel signs on the BMT, "Trains stop at Center of Platform." Two other factors are at work: The labor needed to halve the trains after midnight and to rejoin them at 5:30 a.m.; and the crowds that can occur at any time of night in mid-Manhattan while the same train has light ridership nearing the outer terminals.
Under Mayor Robert Wagner, Jr., I believe, maybe around 1963, we had a crime wave in the subways. Reacting to newspaper prodding, the city announced there would be a uniformed officer at every station in the subway at night and a uniformed officer on every train. (They forgot the havoc when some high schools let the teenagers leave all at once.) At the same time, the city announced that the rear half of each train would be closed in the middle of the night. As I recall it, within a year, the police presence diminished and the trains were opened their full length, just because of the loading problems midtown.
All the above is subect to faulty memory.