Typically, temperature does affect elastomer stiffness (as temperature drops, stiffness increases). 

In the ShockStop, we use a special material for our elastomers that slight stiffening would only begin to occur well below freezing and the material remains flexible well past -40 deg F. 

While you would see some stiffening at very cold temperatures, we expect it to be minor and only slightly reduce the magnitude of shock absorption. In general, we would not recommend riding when the temperature is below -40 deg F.