Dealer completed replacement of block heater and power supply cord this morning, Service manager did state to me that these duramax engines really do not need plugging in until -15 to -20 degrees. That blew my mind, however this is my first diesel so I am learning throughout this process.
Mine have always 'lived' outside and get used everyday.
I regularly start them down to 0*F or just below.
If I was expecting to need to start at -15*F or below I'd unwrap the cord and find a way to plug it in. That said I'm sure we've had members post about unaided starting at those temps.
Back when the LML was new, GM bragging about how quick and easily they would start, unaided, with temps in the 0*F range.
Owners manual does say 'they will start easier' if plugged in when below 0*F.
This is the 2018 diesel supplement, but the others are similar: Engine Heater
If equipped, the engine heater can provide easier starting in cold weather conditions at or below −18 °C (0 °F).
They also included a warning about not using the block heater in a garage....even in 2018.....before the recent recall.... Warning
Do not plug in the engine block heater while the vehicle is parked in a garage or under a carport.
Property damage or personal injury may result. Always park the vehicle in a clear open area away from buildings or structures.
EDIT to add:
2020 supplement has same recommendation and same warning...