Apple has begun sending out notices to some iTunes and App Store customers advising them of some upcoming payment security changes that will limit compatibility with older iOS, macOS, and Apple TV software versions. The emails appears to have been sent out only to those users that Apple has identified as still using one of the older versions, and specifically notes that users of iOS 4.3.5 or earlier, macOS 10.8.5 (Mountain Lion) or earlier, and Apple TV Software 4.4.4 or earlier will no longer be able to change their payment information from those devices.
The change also appears to only affect the ability to change payment methods, not to access or make purchases the store itself from these older devices. Considering that the listed operating system versions are all over five years old, and that only the original 2007 iPhone, the 2008 iPhone 3G, and the first two generations of iPod touch devices are incapable of being upgraded to iOS 5, the problem seems very unlikely to impact most iTunes or App Store users.
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