I found this:
“When the root zone temperature of any plant is below 13-14 degrees Celsius, there is zero activity in the plant whatsoever. This means that even if the soil were soaked with nutrients, the plant is not going to take anything because it is too cold.
14C = 57F
I also found that at 40F cannabis roots start to die.
And this
The Importance of Creating a Healthy Root Zone for Cannabis
Root zone management is one of the most crucial aspects of cannabis cultivation. After all, the roots are the primary route for essential water and nutrients to reach plant tissues.
The root system will develop and function best when kept within a specific temperature range (usually between 70 and 78 F, 18-25 C). A master grower should monitor RZT closely, just as he/she needs to monitor and regulate air temperatures. All parts of the plant are interconnected, and nutrient absorption issues can occur in properly fed plants if the root zone temperatures stray outside the correct temperature range for too long.
Without a healthy root system, your cannabis plants can never reach their full potential. For a heavy harvest full of spectacular buds, you’ll want to do everything you can to support the health of your plants’ roots.
Cannabis roots are happiest at around 24°C, (76F) respiring and growing most during the night and early morning hours.
But I found no studies on the practice of cold roots at harvest.
IMO this is not something that makes sense to improve harvest. Open to a real study on this tho.