NASA has been asked this question before, and they cannot confirm that the Voyager was launched with Scapa Flow sourced steel; but, the story behind why it might have been included is interesting (although it seems like a chain of possibilities rather than proof that any such thing happened).
The process of making steel involves superheating metal with forced-air blown furnaces. The air post-1945 is contaminated with trace radioactive elements. For some very sensitive types of equipment, like Geiger counters, the contamination from the furnace air sufficiently taints the steel, giving the steel a slight radioactive quality. The Scapa Flow scuttled fleet is a large amount of pre-1945 steel, and it is cheaper to rework it than to create steel in a scrubbed environment.
However, the air of today is not as contaminated as the air of 1946. Today, there are many batches of steel produced which is of suffienctly low radiation that they can be used without the need to salvage ancient battleships.
In addition, the Scapa Flow have mostly already been scrapped. The source of cheap Low Radiation steel by salvage operation has some sources indicating that Scapa Flow was mostly salvaged back in the 1930's although some steel was still being salvaged as late as the early 1970's.
Finally, many of the space craft and space craft components are not sensitive to low levels of background radiation. There are many examples of space craft made with modern steel.
The Voyager space craft did not launch with Geiger counters. One page details the sensors: television cameras, infrared and ultraviolet sensors, magnetometers, plasma detectors, and cosmic-ray and charged-particle sensors. In addition, the fuel for the craft is a radioisotope, which would complicate shielding should a Geiger counter be required.
While it is still possible that the craft was made with low background steel, it seems highly unlikely:
- Low Background Steel is used in applications where background radiation interferes with the mission.
- Low Background Steel is not exclusively used in space craft.
- The majority of Scapa Flow was salvaged 30 years before Voyager construction.
- At least 15 years had passed, with a corresponding reduction in background radiation prior to the construction of Voyager.
- Voyager shipped with radiation sources on board, to power the craft.
- Voyager did not carry Geiger counters, or devices primarily designed to test for radioactive radiation.
- It seems highly unlikely that one would source Low Background radiation steel from a reduced salvage area for a space craft that doesn't detect radiation and carries an onboard radioactive electrical generation plant.
It seems far more likely that:
- Scapa Flow salvagers know the value of Low Background Steel.
- They are happy to detail the applications.
- Sensors are a primary application. Space is the coolest place they are deployed.
- Voyager has sensors (but the are not the sensors that require this steel) and is deployed in space.
- The similarities between points 3 and 4 are enough that a person will make the mistake of correlation of a hypothetical situation with a real world event to conclude that the hypothetical situation occurred as it was a requirement; when in fact it wasn't a requirement for many other reasons.