You know the problem. You used a Google account to login to some Google app on iOS. Now you want to get rid of this login for some reason. Just logging out won’t help as the apps remember the used accounts.
Deleting all the Google apps won’t help either because reinstalling will surface the logins again. Resetting the iOS advertising identifier to avoid Google being able to fetch the old login data from their servers won’t help either.
So it seems the logins are neither stored on this device nor on Google’s servers but most likely in some sort of iCloud app storage facility which you can’t delete. Apps have the ability to use iCloud storage to store settings for free.
That means the Google logins will persist for this iCloud login. Quite annoying especially if you don’t want to link different Goggle accounts because of privacy reasons. But there is an easy way to delete the stored Google logins.
Installing Gmail gives the option to remove a login from the account settings page in Gmail iOS on either iPhone or iPad. By doing this all logins will be removed from all Google apps including YouTube, Google Maps, Google Chrome and others.
To remove a login tap Accounts > Edit > Remove for all the logins you want to remove. Removing a Google login from one app deletes it for all installed Google apps on this device.
The same goes for Google Drive, Google Hangouts and Google Calendar. These apps can manage accounts more thoroughly than the rest of the Google apps. Maybe Google will enhance the other apps in the future as well.
It took me some time to figure this out as I didn’t have one of those apps installed on my devices. The Google apps I used and where I wanted to get rid of the accounts don’t have this ability.