Apple plans to shut down iCloud and other Apple services for older OS releases according to @StellaFudge.
iOS 11 – 11.2.6, macOS 10.13 – 10.13.3, watchOS 4 – 4.2.3 and tvOS 11 – 11.2.6 are expected to loose access to iCloud services later this year. No formal confirmation from Apple is yet issued though.
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