How is 4:3 content HD?

I don't think HD refers to the aspect ratio but the number of pixels in the image (usually 720 or more vertically). Don't expect a TV series that old to be true HD because if it was shot on a TV camera it has a limited number of lines anyway. Now you can do anything with encoder settings and you can take a pretty low quality image and encode it at high quality. You just end up with a high resolution image of the original quality picture.

I have seen old TV shows with widescreen format but I suspect there they have had to cut off the top and/or bottom of the original picture to get one of that format.

As for getting Apple to add the information they would also have to get it from the vendor:

http://www.apple.com/feedback/itunesapp.html

HD refers to the pixel height, not widescreen vs full screen. That show was produced for 4:3, and in many cases, especially for TV, they did not create a widescreen version.

HD technically is anything that is 720p (720 pixels high, the p stands for pixels) or higher, though it most commonly refers to 720p or 1080p. DVD quality is 480p.

4K or UHD is measured by pixel width rather than by height. 4K is 4096 pixels wide (3840 if 16:9).

Video quality is not tied to the Aspect Ratio. It can be HD in 4:3.

The Aspect Ratio determines the shape of the video not the quality.

HD simply means its in higher quality that SD however much that may be.

In the case of a Tv show filmed in the 90's with a square aspect ratio, there's no way short of cropping the image and zooming in to create a widescreen video.

Quantum Leap was filmed in 4:3, so no way to get 16:9 out of that.

The HD version may have been processed for better quality, but the video shape is not something you can easily change.