That is a good point. Are there any plans to fix up shape keys in the future?
I was thinking of the normals just being a linear interpolation from 0-1, and it would average the various shape keys normals. I didn’t think about the fact that shape keys can go into negatives and beyond a value of 1.
I think an alternative that would fit my and others needs would be to add an option for a shape key not to affect the custom normals. I feel this would be less complex to add.
I imagined something that big would be complex to add in. I think I will try to implement the non affecting custom normals option and put it into a custom build for others to test before trying to get it into master right away.