Does solder really expire?

[Ian.M]

How much do you think flux technology has improved in the last half century?

If you've inherited a roll of solder or had it on the shelf 40 years, you almost certainly have no idea of the original performance of the flux.   If the solder wire surface isn't noticeably oxidised, and you are far enough from any cut or broken end for spitting and popping due to moisture in the flux cores not to be a problem, poorer wetting and flow-ability than modern solder of a similar type may just be due to better modern flux.