In a new agreement between WhatsApp and Google, WhatsApp backups will no longer be counted against Google Drive storage quota. Earlier the WhatsApp backups where eating up the Google Drive storage and the free quota may get used up pretty soon if you exchange a lot of videos and photos over WhatsApp.
However, any WhatsApp backups that have not been updated in more than a year will automatically be removed from storage. This policy will be into effect from November 12, 2018. However some users may see the quota benefits earlier if they are using the beta/testing version of WhatsApp. To avoid the loss of any backups, Google recommends that people manually back up WhatsApp before November 12, 2018.
Recommended Actions to be performed before November 12, 2018
1. Confirm that your current Android WhatsApp client is backed up
2. If you would like to refresh any existing backups, please manually back up your WhatsApp client before October 30, 2018
This wonderful move by Google and WhatsApp helps users get rid of one headache of keeping a separate local backup of WhatsApp media files. But once the media is deleted from the phone it is removed from the backup also. However if user chooses to backup manually, then the whole of the WhatsApp media files can be stored for 1 year full on Google Drive even after deleting from the phone without worrying about the 15GB free quota allocated by Google Drive. But make sure that after deleting the media from the phone, do not initiate the manual backup as it will delete the files from the Google Drive.
Please note that the WhatsApp backups on the Google Drive is not end to end encrypted. Whoever has access to your Google account (inculding the Google team) can view those files.