How long have movies been shot in high definition?

Most movies are shot on film and not captured to video. Because of that, they have never suffered from the constraints of television resolutions. Typically, 35mm and 70mm film have an effective resolution that is higher than either SD or HD television.

When a movie is broadcast, it is converted to a video signal using a tele-cine unit (a film projector and television camera combined into a single device). The television resolution is defined by the camera in the tele-cine unit at the time it is captured to video.

so, film that has been shot before the days of HD television can still be broadcast as HD.