The SCR system is supposed to be able to deal with this. Supposed to. I think VAG's strategy seems to struggle, though.
In theory, the system knows how cold it is (plenty of inputs to find this data in the car), so it just turns on the heater(s) when required and doesn't attempt any DEF dosing when it knows the fluid is frozen. And it tries to keep the lines clear when you shut the engine off (the pump runs backwards to pull any DEF from the delivery line).
The CKRA's have had a lot of post Dieselgate fix SCR issues related to the system's logic for resetting the countdown as well as improper dosing during extended high speed (80+) driving. I am really not sure if they've ever really got it entirely sorted. Which is why, unfortunately, deletes are so popular.
Other than the injector/delivery line leaks, the CRUA and CVCA engines in the 2015 cars seem to have fared better post fix. And the leaks have nothing to do with any changes made anyway.
All the manufacturers seem to have had some teething troubles dealing with SCR in extreme temps. Why VAG continues to struggle with the CKRA I have no idea. I know MB completely revamped their entire SCR system on the Sprinters in 2014, and this dramatically improved their durability. The 2010-2013s were problematic (tank heaters mostly).
It is a great idea in theory. A post-combustion NOx reduction strategy, allowing less EGR, so letting a diesel engine do what a diesel engine does best, and run LEAN, which then also means less soot, so less DPF regens, which all equates to better fuel economy. The fact that the relatively large NMS Passat can come close to an ALH car on the highway MPGs is pretty impressive... it couldn't do that without SCR. But it sucks that VAG's SCR strategy evidently wasn't enough to keep them NOx compliant all the time, and worse, not even able to be improved upon sufficiently to allow the manual CKRA cars to get a pass
But clearly the durability and reliability of the system in general is still quite suspect.