It’s a number of reasons. One I don’t see already mentioned is that Apple and Google require apps to target the latest versions of their OSs and libraries. For example Google released a new version of the Google Play Billing Services library. All apps were required to update to the newest version by mid August (you could request a two month extension). So to the end user it seems like nothing has changed. But under the hood the app is now using the latest apis. This could also apply to non-Google/Apple apis. Maybe a change of the developers own api was necessary.

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