Write them, tweet them, etc. It was a big thing when iOS 7 came out, but people are probably not talking about it as much. Can we replace app icons in iOS like we can for OS X? There are some nice iOS 7 styled IG icons online.
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On a side note, if everyone starts leaving them bad feedback in the app store for the outdated icon, they will probably update it. Reviews directly affect how they're discovered and downloaded, etc.