Here's anecdotal evidence that compressed springs do deteriorate over time:
My late father purchased a new 1963 1 ton GMC truck with extra heavy duty rear springs which jacked the back end up pretty high, until a cab-over camper was loaded onto it, then it was "about right". In 1976, the camper came off to enable the truck to pull a 5th wheel trailer - but the height of the rear end remained "just right", it didn't spring back up.
YMMV...
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