I claim the ornamental design for a door threshold assembly, as shown and described.
FIG. 1 is a perspective view of the door threshold assembly, showing my new design;
FIG. 2 is a front side view thereof;
FIG. 3 is a back side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The evenly spaced broken lines depict portions of the door threshold assembly that form no part of the claimed design. The dot-dash broken lines depict the bounds of the claim, and form no part of the claimed design. The drawings contain a symbolic break. The portion between the break lines faints no part of the claimed design.