Interesting. My house in Green Bay WI is a 1973 vintage tri-level, with two bedrooms and the main bath directly over the unheated built in 2-car garage. Other than the OH garage door, the other three walls of the garage are insulated, at least to 1973 common practice. I remodeled the main bathroom back in 2018, which included removing a cast iron tub sunk into the bathroom floor (over the garage) and replacing it with a floor level corner shower/tub. Winter of 2018 it got real cold, like -20 F air temp, and the drain line for the tub did freeze. My solution was to create an air passge from the basement which is always quite warm (furnace, hot water heater) to the area below the shower/tub. No issues since.
When I built in the framing in the bathroom floor to replace the tub I took out, I found the joists over the garage were 2x12. So I heavily insulated that area where the tub had been. Makes me wonder what codes apply in WI for the tens of thousands Bi-level and Tri-level homes with living spaces over unheated garages.