Release 10.5.9 introduces a change to how storage is presented in the UI.
With 10.5.9, Backup Storage Usage may appear to indicate less available storage, though this is only a matter of presentation and does not reflect an actual change in the available storage of the backup appliance.
The available licensed capacity is now shown, where previously the total physical capacity was shown. This is a more accurate depiction of how much storage the appliance is using and how much available storage remains, as the license ultimately dictates the appliance's capabilities.
It is important to note, there are no changes in how storage is controlled, how retention is maintained or how or when backups are purged. The only change is in how storage is presented in the UI; the used licensed storage capacity is represented rather than the used available raw storage.