A few years back I bought a pound roll of fine Ersin multicore. I do so
little soldering nowadays that this roll is likely to outlast me, even if I
live another 20 years.

The flux isn't exposed to air, so the only way it can deteriorate is the
normal thermal degradation that any organic substance undergoes. I suspect
this occurs very slowly.

Again, we turn to the Lady from Philadelphia for advice: "Why don't you just
try soldering few test joints and seeing how well the solder works?" Duh.

PS: I'm bothered that electronic products increasingly have RoHS labels on
the box, and statements that they use non-lead solder. I really /don't/
believe that a lot of lead is leaching from waste sites into the water
supply, so we aren't likely to see much, if any, reduction in the amount of
lead (and lead compounds) in our water supply (is there any hard science on
this?), while the reliability of electronic products is likely to decline.

I like to say that there is no such thing as a hazardous substance. The
hazard occurs from how the substance is used (or misused).