Google can automatically delete the Android backup for those devices that have been left unused for two months, however, notifies before doing so.
A Reddit user had posted on website Android Police that Google simply deleted the entire backup to his data saved in Drive without any prior notification. However, Google has clearly mentioned in its terms and conditions that if the device is left unused for two weeks then it gives notification of expiration date below the backup.
Google backup policy says "Your backup will remain as long as you use your device. If you don't use your device for 2 weeks, you may see an expiration date below your backup.
So, if a device is unused for two weeks, Google Drive will show a countdown or the number of days you have to save your data.
Google basically wants users to use the device. But if the device is left inactive for two months, Google will automatically delete the backup.