It makes no difference whether the film was nitrate or safety, the emulsion producing the image was silver, supported by gelatin.
The base is irrelevant in silver recovery. This is still done today with polyester based film (Kodak's trade name Estar).
Color film is a different story, as the silver is dissolved out during processing, replaced by dyes. Silver is recovered from the used processing solutions.
Since heavy use of color film came in around the same time as safety base, silver recovery from used film decreased.
