I lived and flew in Alaska for over 40 years, 18+ years in Fairbanks where -20F was my cutoff without a really good reason. I did do one multi day SAR in a T-34 where it was -55F and my lunch would be frozen 2 hours after take off unless I kept it inside my clothing. I had my C-206 out one day at -45F but that borders on sheer stupidity. Doors didn't want to close and plastic cracked but the real problem is survivability if you go down. In all likelihood you will be injured in the landing(crash) and in that country you will spend at least one night before rescue.
The reason for the -20F cutoff was that you would be looking at a long time between flights if you wait for anything much warmer. Winter flying in that country can be very rewarding with nearly unlimited visibility, outstanding aircraft performance and smooth air. And, there was usually an inversion so a couple of thousand feet up it would be significantly warmer. But, that won't help if you get to spend some unexpected time in the middle of nowhere.
The reason for the -20F cutoff was that you would be looking at a long time between flights if you wait for anything much warmer. Winter flying in that country can be very rewarding with nearly unlimited visibility, outstanding aircraft performance and smooth air. And, there was usually an inversion so a couple of thousand feet up it would be significantly warmer. But, that won't help if you get to spend some unexpected time in the middle of nowhere.