What is the minimum operating temperature of chemical hand warmers? I have been looking at chemical hand warmers such as Hot Hands, Grabber and HotSnapZ. Unfortunately, I cannot find a minimum operating temperature for these types of devices. Logic tells us that they won't work at -273°C and will probably work at 0°C, but I'd like to have an idea of where the cutoff lies between them.

From Wikipedia here and here, it looks like they all contain some amount of saltwater. The lowest freezing temperature I can find for saltwater at saturation (26%) is -21°C. Is it reasonable to assume that a frozen hand warmer will not work? Would the other components in the hand warmer lower the freezing temperature even more?

TL;DR
What is the minimum temperature at which a chemical hand warmer can still provide heat?
They are often made of the crystallic $\ce{CH3COONa.3H2O}$, that dissolves in its own crystal water, when heated up.

As the cooled down liquid state is supercooled solution, it is hard to predict, at what temperature it would spontaneously solidify.

It will provide heat in any case. But as it warms up itself typically by  $\pu{40-50^{\circ}C}$, it would barely warm itself to room temperature, if it was previously cooled down to e.g.  $\pu{-60^{\circ}C}$.

The air activated heaters, based on oxidation of fine metal powders (thanks @MaxW ), may have problem with too slow initial reaction when too cold. So they may need to be kept at pocket temperature for some time before activation.