Another year round commuter here, where my coldest 30 minute commute was 5 F (-15 C). And my coldest Imperial Century (for fun they said???) was on New Years Day in 2018 with the local Rocky Mountain Rando club, where the start was at 12 F, was below 20 F for the first 50 miles, and never went above 27 F for the final 50 miles. That one was a character builder: https://www.strava.com/activities/1336643971

Nowadays, with all those hardcore shenanigans out of my system, ~25 F (in full sun) is my lower limit for a ride of any duration. Any colder than that requires a base layer under the winter bibpants, which makes the riding less than comfortable, due to heat/sweat buildup.