Modern cartridges loaded in Europe have steel bases that have been given a brass wash. That they are rusted indicates that these are steel based cartridges.

I presume that as you mention using these on wildfowl that they are loaded with other than lead shot? If Bismuth or Tungsten Matrix I don't see a problem. Other than that the bases might be so rusted that they come apart on firing!

But if these cartridges are loaded with steel shot and have rusted inside as well you'll be firing a steel slug through your gun. With predictable results to it.

If in doubt....don't!

The only other argument - from the time of paper cases really - is that case (and powder) dry out. Making the former weaker and the latter stronger. But you'd pretty much have to have kept them in a warm dry room for many, many years for that ever to be an issue.