This is rather too simplistic. For starters not all developer ingredients are salts. Secondly, whilst it's generally true that dissolved substances will lower the freezing point of water, the bigger potential issue is precipitation of the ingredients due to lowering of the water's saturation point as it cools.
Having said that, it is likely that any such precipitation is reversible once the mixture is warmed up again. I imagine also that most commercial developers have been designed to tolerate a range of storage temperatures without such things becoming an issue.
In conclusion then, not something to worry about.