Original 747 with JT9D engine's had turbine reversers and the L/E devices retracted to stop them from being buffeted by the hot reverser flow and also to keep the hot and somewhat corrosive and dirty, sooty gases out of the leading edge cavity. When they gave up trying to keep the turbine reverser operative and issued an SB to delete/deactivate them permanently the auto retract function on reverse was deleted as well. Later, Boeing reinstated the auto retract on reverse as I think there were still fatigue issues from buffeting with some engine types.
707 retacted the L/E devices for same reason - keep the soot and hot gases out of the leading edge cavity.