Below freezing seems to be the standard rule[1]. One typical solution is to use the charging current to heat up the battery instead until it's above freezing, then start the charging.

You could of course also use some of the charge from the battery itself to heat it up.

[1]: https://relionbattery.com/blog/lithium-battery-cold-weather

Anything below 0C is problematic for LiFePo4.

I have a small solar powered device (Pi Zero controlling an antenna switch) in my backyard that I use a little mechanical temperature switch to turn on and off a teeny tiny mat heater attached to the battery. It's all in an enclosure that's water tight, so I doubt it got below 0 during the day in there in the first place. We didn't get temperatures below 0C this winter in the NE USA (which is not ordinary), and even with some dips below 0 the battery seems fine.