Unloaded springs don't deteriorate, for sure, but with loaded mag springs, the story is different. It depends on how the spring is used and the design of the magazines.
A very erudite discussion. Thanks very much for your informative response. Guns are very complex to most of us that like, have, and use them; and would like to fix things that go wrong. I worked on the magazine for my AMT .45 Backup tonight, and it seems to be feeding reliably. Can't wait to fire a few rounds out of it!