I can guarntee you that protien doesn’t degrade as long as it’s kept dry.Searching around I found 5lb containers of Casein for $30 and Whey for $20 so I loaded up. Kijiji believe it or not for some of it!! My belief in "Best Before Date" depends on what the item is. For protein powder I believe it's just a suggestion
We use isolate whey and isolated soy protein in certain types of sausage. Right now I’m finishing of a bag of isolated soy protein that is 6 years old. Binds exactly the same way as it did new.
Just keep it dry.