Please elaborate on what was added by whom and not used?
I believe that this was imposed on the program by the US Airforce due to the need to collect spy satellites for service and put large payloads into polar orbit.
I think that the polar orbit requirement was never used, and had a very significant impact.
One of the things that was to enable the polar orbit was a new set of solid rocket boosters, the 'Filament-wound' models that had a lighter weight outer case, and more propellent.
The book Riding Rockets was written by an astronaut who was in the middle of prep on that discovery mission when Challenger happened, and has a few details on the proposed polar orbit launch.