Former subscribers of MobileMe, the $99/year iCloud predecessor, received 20 GB complimentary storage upgrades when making the switch to iCloud. Apple additionally extended this upgrade until September 30, 2013. Now, Apple is reminding those users, via email, that the complimentary storage will come to an end shortly:
As a thank you for being a former MobileMe member, you received a 20 GB complimentary storage upgrade when you moved to iCloud. Your upgrade expires on September 30, 2013.
When it expires, your iCloud storage will be automatically adjusted to the free 5 GB plan. Note that you are currently using xx.xx GB of storage. If you exceed your storage plan on September 30, 2013, iCloud Backup, Documents in the Cloud, and iCloud Mail will temporarily stop working.
To continue using these iCloud features without interruption, reduce the amount of iCloud storage you are using or purchase a storage plan by September 30, 2013.
For more information, see this article.
As our own Benjamin Mayo notes, this will probably be a busy day for Apple’s support representatives as they work with the customers who are over the 5 GB free storage size. Much of the iCloud storage is usually taken up by iCloud Backups which can be deleted through System Preferences > iCloud > Manage…