That was an option that was recently removed. We did a major review of
the various optional settings offered and tried to find options that
could be removed with minimal overall impact the the user base.
Permanently displaying the BCC field was an option that was rarely
used, and since it can be revealed without a page refresh, it was one
of the casualties.
We are definitely not happy about the impact it has on the users who
relied on it regularly, but we hope that the inconvenience is small
enough when compared to our overall goal of streamlining the products.