Metal fatigue is not the issue. Nobody can cycle those mag spring fast enough, close to its natural frequency and for long enough duration, for that to be a problem.
Mag springs do lose a bit of their "shape memory" in normal operation. To see this, you'll have to take the spring out of the magazine and measure the "free length". Do that when the mag is brand new unused. Then repeat some time later after constant usage. More than likely, you will see a slight - but noticeable - difference in free length. Whether this difference will affect operation of the magazine is an entirely different issue.
Spring steel, when formed into the shape of mag spring in the mfg process, will carry some residual stress. Normal operation of the mag in use will relief some of that stress. This is another reason that folks notice a mag spring taking a "set" after some usage. It it normal and do not typically affect operation.
