Prevent google backup of contacts?
As said already, backup of contacts by Google does not respect the general Settings&gt;General&gt;Backup&gt;Google_backup&gt;Backup_to_Google_Drive = off. This problem may occur only on LG G6, or in Android Oreo in general. 

However, I succeeded to prevent the contact backup by disabling Google synchronization services in Settings&gt;Apps&gt;Show_System&gt;choose_app&gt;disable:


Google Contacts Sync 
Google Calendar Sync 
Google Backup Transport


Now I hope that they won't get enabled again upon a firmware update.

Edit: Evidently, Google has more problems with collecting personal data than just contacts, see for instance location-tracking.