Even on a military budget, it's still easier to make a very large piece of film than a very large digital sensor, wide area panoramic imaging appears to be the niche this camera occupied. With the images being 5 inches by more than 6 feet long, I expect the film was driven past the lens at a rate compensated for image motion as the plane flies. I'm sure it's digitized quickly after development for image processing, so it's good this is in the Hybrid forum.
They don't fly the U-2 in places it could be attacked, obviously (not over superpowers). The NASA variants of the U-2 are still flying for scientific research and civilian uses, including carrying infrared sensors. A couple of years ago, I saw one of the NASA U-2's in flight condition in a NASA hangar in Palmdale, CA. No pictures allowed in the hangar unfortunately, although there wasn't anything secret in it (I don't have any restricted access).
Just down the street, there is an aircraft museum with a number of unusual airplanes, including two SR-71s. Here's a couple of pictures. In the second one, they've put an engine on display in case you want to try reverse engineering it. The museum was closed that day, but I don't think they have a U-2 anyway.