I haven't toured much below 0C daytime/ -10C overnight.
The days are a somewhat larger constraint. I find there is definite boundary around -5C where despite gloves/mittens my extremities are susceptible to a cold frost nip and I end up stopping and reheating multiple times. So not a big issue if it froze overnight if it warms up past freezing not long after I am on the road. Overnight temperatures are more a case of bringing the right sleeping bag and otherwise blocking wind/cold from getting in. Getting that extra sleeping bag when also dealing with warmer temperatures on an extended tour means I sometimes end up with my normal bag + an second overbag that I use for colder nights. I used this for example in highlands of Peru/Bolivia where it got quite cold, but tropical areas of South America were warmer.
I lived in Colorado without an automobile for eight winters and commuted in colder temperatures down to about -20C but those were relatively short commutes (7km) so I could dress appropriately and had a warm office/home waiting at the other end.