The main reason for a slope back tank is for visibility when backing. I have wondered if when the Class 125's were built the turntables at Marshall Pass and Sargents were still 50' and unable to turn the engines. In which case they would had to back to either a wye or a longer table. Even with a slope back tank, backing that distance would have been a serious pain in the butt.
Photos show the slope back tanks lasted after the engines were simpled. I seem to remember a date of around 1911 on the larger tender drawings.