I have never heard that ultrasonic cleaning could damage a leaf or coil spring, and I can't imagine any reason that it would.
However, some people utilize a cleaning method called electrolysis to remove rust from gun parts. It works quite well to remove rust, and some guys swear by it. But electrolysis uses a DC electric current which causes the water in the solution to break down into oxygen and hydrogen. Because of this, it can and does impart hydrogen atoms into the molecular lattice of the steel, and causes a condition known as hydrogen embrittlement. This can cause parts like springs to crack under otherwise normal loading.
The greatest factor in eventual failure of leaf (or coil) springs is micro-pitting caused by rust, assuming the springs were made, polished, hardened, and tempered correctly. It really has little to do with how old a spring is or how many times it has been flexed. These tiny pits create stress risers that are points where cracking and fractures are initiated.
And that reminds me... it's time to spray the torsion springs on my overhead garage doors with a rust inhibitor.
