Here's what the email says:
We discovered a bug in the recent iOS 10.3 software update that impacted a small number of iCloud users. This may have inadvertently reenabled some iCloud services that you had previously disabled on your device.
We suggest you go to iCloud settings on your iOS device to make sure that only the services you'd like to use are enabled.
Learn more about how to manage your iCloud settings or contact AppleCare with any questions.
The iCloud team
MacRumors was able to confirm that bug has been fixed in iOS 10.3.1.
According to Apple, iCloud Photo Library, iCloud Keychain, and Find My iPhone were not affected by the issue. The bug has been fixed in the iOS 10.3.1 update, so those who have not yet updated to iOS 10.3 but choose to update to iOS 10.3.1 will not be impacted by the bug. Apple has not shared how many users were affected by the bug but says not everyone who updated to iOS 10.3 will experience it.
You can double check the toggles for your iCloud services by navigating to the Settings app and choosing iCloud from the Settings menu.