Torsion bars

Technically, the bars do not get weak.

In fact, the spring rate does NOT change as the bars age. Those horribly soft bars were exactly like that from the factory.

They do sag, so, as time passes the car will get lower, and need more turns on the torsion bar adjusters to maintain ride height. Similar to leaf springs- the spring rate doesn't change, but they lose their arch over time.

But these cars really were that horribly undersprung from the factory.