How to prevent shifting issues due to derailleur or cables freezing in cold weather
Clean and dry (hair-dryer) the derailleur. Relube. Check. If necessary take out the cables, chase the water from the housings by pressing thin oil with a syringe. Put the cables back in or replace with new ones. It's always a good idea to change the cables before winter if you've been riding through rain in the good season. And change them again at the beginning of the good season!