The ornamental design for a threshold for a slidingsystem, as shown and described.
FIG. 1 is a perspective view of a threshold for a sliding system;
FIG. 2 is a front or back view thereof;
FIG. 3 is a left or right view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a perspective view thereof with a floor and a sliding door or window in broken lines for illustrative purposes only and form no part of the claimed design.