Apple is sending out emails to a small number of iCloud users informing them of a particular bug in iOS 10.3. The bug evidently causes some iCloud services that were previously disabled to be re-enabled.
From the email, obtained by MacRumors:
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.
Outside of "small", it's unknown at this time how many iCloud users this may have impacted.
You can check which iCloud services are enabled on your iPhone or iPad with iOS 10.3 by heading to Settings, then tapping the Apple ID banner at the top of the Settings page and tapping iCloud.