As I have already said, the mother's maiden name was only added to the original indexes from 1929 onwards, and you are not allowed to view online any birth less than 100 years ago.
I imagine that SP has plans to add the mother's maiden surname to the deaths index once they have completed adding it to all the deaths, and that adding the item to the search parameters is part of the preparation for that.
It would not surprise me if, when it goes live, searching using mother's maiden surname is restricted to births over 100 years ago.