Storing the battery cold isn't the end of the world.
Ideally though it shouldn't be too cold when you charge it. You'll also get less performance from it if you ride the bike and the battery is cold to start with.
Cold storage shouldn't really have much impact on life span. In theory it can even extend it. In reality cold often also means moist, so that could have a detrimental effect of life span - they prefer dry !
If you Google it Shimano say down to 0c is OK for charging and using, although you will have reduced performance - that will go back to normal once temperatures rise. Actual limits stated on their .pdf are -10c to 50c for discharge, 0 to 40c for charging, -20c to 60c for storage.
Generic ebike advice on other sites state you shouldn't charge below 5c but it seems unclear if this is because the battery charges at a slower rate due to increased resistance, or if it actually harms it permanently.