Is it safe to use narrow but short PCB traces for high current flow?
Copper foil will move the heat vertically into the air or into underlying traces or planes through the FR-4.

And the copper foil will move the heat laterally through the trace. The lateral thermal_resistance of 1 ounce/foot^2 foil is 70 degree Cent per watt per square.

A trace of 20 mil by 200 mils has 10 squares, and end-to-end thermal resistance is 10 * 70 = 700 degree Centigrade per watt.

From middle of the 200 mils to either end will be 350 degree Centigrade per watt.

Assuming heat is uniformly generated (a wrong assumption, because copper has 0.4% per degree Cent temperature coefficient) and that heat only flows along the trace, you'll get a parabolic rise in temperature from either end to middle of that 0.2" trace. For simplicity, assume you have 2 milliiWatts of heat generated in the entire 0.2" long (20 mil wide) trace. Assume 1mW needs to flow through 0.1" of trace, from middle to end. The R_thermal is 350 degree Centigrade per watt, thus only 3.5 degree Cent rise.

Summary: for 20 mil wide trace, 200 mils long (0.2"), with 2 milliWatts dissipated in that trace, the worst case rise will be 350degreeC * 1mW or only 3.5 degree Cent.