You are right; we had stated in our old technical data sheet that dry yeast should not be frozen. However in the last 3 years we had a project running to optimize storage conditions for dried yeast. We stored several lots of vacuum packed dried yeast at room temperature, in a refrigerator and in a freezer and determined viability, vitality and fermentation performance every 3 month for two years. The results indicated that freezing the yeast does not harm the yeast but is even better than storing the yeast at room temperature. BUT for all storage temperatures it is very important that the yeast is still vacuum sealed. Air/oxygen is doing more damage to the yeast than any difference in storage temperature.
Forbes & Tobias