Stan's sealant into my freezer, which is set to 2 degrees F. The next day, it was a bit sluggish, but not frozen solid. My guess is that it's effectiveness would be reduced at this temperature, but entirely gone.
Fortunately, I am not in the habit of cycling at those temperatures, hence the freezer test. I am confident my sealant will continue to work in the 10-20 degree area that represents the limits of my personal operating range.
I know that some people, god bless them, regularly go riding in sub-zero temperatures. The last thing they want on such a ride is a flat. One interesting thought I had was to dilute the sealant with some alcohol to lower it's freezing point. Vodka and Stan's anyone? Heck, the Slime people purposely made their Slime Pro smell like a Pina Colada! Seriously, I would probably use over the counter rubbing alcohol as it is more concentrated, less expensive, and can be purchased without ID.
I have not yet tried other sealants such as Slime and CaffeLatex in my tires, much less at extreme cold temperatures. Does anybody out there want to chime in with their experiences with different sealants at different temperatures?