We might as well add solder to the list of things that will still be around after a nuclear blast. I think everyone that has been in a technical support position at a solder company has gotten at least one call from a customer with really, really old material. Of course, these calls aren’t to exclaim how old the solder is before they throw it out – it is to determine if the expired material can still be used.
If you need to solder a couple pipes together at home, that Sn/Pb wire from the 1980’s will probably work fine. I even used flux that old around the house – but it has no place in the demanding world of device packaging. Although solder companies tend to err on the conservative side for shelf life values, it really comes down to the application at hand. Old flux can cause variation in dispense amount and solderability due to reactions in the material over time. It is always a good practice to follow shelf life recommendations for soldering materials.