There doesn't seem to be a good answer. Zero used to say leave it plugged in, because the risk of the battery getting very very low was too great, and holding the battery near 0 is one of the worst things you can do to it. Then they said, ok, hold it at 60%, that's better. Presumably someone thought that if you didn't keep up with it that was your fault and the warranty was voided. My thought is if you might forget to check on it every few weeks, its probably still better to leave it plugged in.
Now the new models have long term storage mode so this is moot for new bikes. I am hopeful they will push that in a software update to the rest of us. In the meantime, I don't think any of the options available to us cold-weather dwellers are great, but the "best" thing is to hold it near 60%.
My concern is that charging the battery below freezing, and especially far below freezing, is supposed to not be too great for it, so do we come out ahead keeping it at 60% if that includes occasional charging in very cold conditions.
Ultimately, that is what I do. From what I've read on the topic, as long as you don't keep it near 0 for a long time, nothing you do is going to be much worse for the battery than the natural aging with time anyway, so I don't worry about it.