We’ve detected that your app is using an old version of the Google Play developer API - Fastlane issue
You might be getting this error in the Play Console:
We’ve detected that your app is using an old version of the Google Play developer API. From 1 December 2019, versions 1 and 2 of this API will no longer be available. Update to version 3 before this date.
If so, most chances are that you're using fastlane to manage your Play Store releases. Currently, fastlane is using Google Play API version 2, which will reach EOL on December 2019 (read more about it here)
There is an open issue with fastlane, meaning supply (the fastlane component responsible for uploading to the Play Store) it will hopefully be migrated to API Version 3 by the end of 2019.
So you can safely ignore the warning, and just make sure to update fastlane before the end of the year.