[sandworm101]

There were superfilms, film stock sensitive in the infrared/UV spectrum and filters to optimize such film. There was also film sensitive only to very specific colors. Releasing images from these, at any resolution, would indeed give away much of the program's abilities/goals.

The ability to more easily declassify film stock is due less to the technology and more to the bureaucracy within intel communities. The film stock is owned by a single agency and so the declassification authority is relatively straightforwards. Digital imagery is shared instantly with a host of different agencies, many of whom still do not talk to each other regularly, and is stored in countless archives. Declassifying a digital file is therefore an administrative burden in comparison to a roll of film kept by a specific agency.

[varjag]

These were all known since 1920s-1960s and very little progress has been made since. Everything you named is bog standard film photography practices.

[sandworm101]

Yes but the use of a particular technique in a particular location/time would divulge the specific collection goals of an operation, something that often remains classified long after the operation itself has been acknowledged. So while the existence of UV film is no secret, knowledge that it was being employed over a specific site at a specific time can be.