I really don't think that's necessarily true (in an application in which the cable won't be flexed). For a start, there are plenty of materials which aren't appreciably flexible at any (sensible!) temperature but which are highly stable at very low temperatures.
The problem with flex is surely when it is being used 'as flexible' (not in fixed wiring). In that situation, when it becomes cold enough to become brittle, flexing will rapidly result in damage/deterioration. However, it was just a matter of interest - and the answer is of no particular importance me.
FWIW, I've got some standard PVC T&E outdoors ('fixed' and never disturbed) which has been subjected to the worst of low temperatures which the UK climate has thrown at it (including some pretty long very cold spells) over the past 25 years, and is still fine - both visually and IR-wise. If I 'unfixed' and tried to bend it, that obviously might be a very different story.
Kind Regards, John
