What is the critical temperature when (batch) desoldering?
tl;dr  : It depends.

I think you are referring to the method of heating things up with a hot air gun, and then shaking the pcb so everything falls of, or a variation of this.

First of all, remember that the process of actually soldering SMT PCBs is quite similar. Stuff goes into a reflow oven, heats up until solder paste melts to liquid solder, then cools down. 

But of course, this is a much more uncontrolled thing when using a heat gun. It will depend on:


How much the heat gun heats the parts (here really the inside temperature matters most)
How much humidity has accumulated in the parts
For how long the parts are exposed to the temperature
The specific thermal properties of a part.


For the last point, you can look at a datasheet of a specific part, especially at the reflow profile. Controlling temperature and exposure time is next to impossible with a heatgun, and when the parts are sensitive to being heated up while containing humidity, they are toast.

So it all depends and is mostly luck and statistics. When doing it by people with some experience in the process, the yield of functional parts can be way over 80%.