I've observed cabbage surviving through an entire winter here in Zone 5. The coldest day last year got to about -19F but everything was buried by snow so it's likely the plants were protected.

In any case, they can survive extreme cold, but may end up being pretty tattered and unappetizing.

I put broccoli and cauliflower in in October also. It's usually not a problem here in So. Oregon. This is our "rainy" season. We haven't had rain in months! The temp has been in the low-mid 20's at night for that long. Beautiful cold days. I admit they look terrible in the morning but by noonish they perk back up. I'm just not sure how long they can take the temps. They are starting to look pretty frazzled. Every day they don't perk up quite as much as the day before. Normally we would be eating it by now but I guess because of the cold, they aren't growing very fast at all. I'm sure a cover would help I just don't have any spare. Wasn't expecting to have to cover cole crops in December!