This is such old news I'm surprised the question comes up any more.
It's not old news if you've never seen or heard the subject discussed before.
Several years back I bought a M1 Carbine pouch with two loaded magazines. By headstamp of the cartridges, maker's date on the pouch, rust on the magazines and verdigris on the cartridges, vintage 1944. Looked like they'd been kept as a souvenir for 50 odd years. The magazines cleaned up nicely and function perfectly in my carbine.
I was gifted a Phoenix Raven MP25 over 30 years ago, picked up a spare magazine, kept the magazines loaded, took the gun out once a year, fired the magazines empty, reloaded the magazines, put it up for the next year's test. The magazines have not lost strength.
Bought a discontinued CBC 7022 rifle at Walmart for $60, then bought four magazines. I have kept the five magazines loaded with the rifle and ready to go if I have an off chance to go to the rifle range o country for plinking. The magazines work fine ever time and show no sign of weakening.
Raven MP25 and CBC 7022 are low dollar guns, but if their magazine springs last fully loaded over decades, then most magazine strings ought to hold strength left loaded.
It seems counter intuitive, but good magazine springs uncompressed or fully compressed don't lose strength. They do wear out by flexing, compressing and decompressing.