It has been discovered that when you try to clear all cookies in the Chrome browser, every cookie will be deleted except for authentication cookies created by Google. This means that after clearing cookies, you will be logged out of every site that you are currently logged into, except for Google.
Cookies are small data files stored on a users computer that are used by sites to store information that persist even when you close the browser and reopen it at a later time. As cookies can also be used to track you on the web or your activity on a site, they can be considered a privacy risk. Therefore it is not uncommon for users to routinely clear all cookies on their machines and expect those cookies to be removed.
This issue of Google's cookies not being deleted was first discovered when Christoph Tavan when into Chrome 69's "Clear browsing data" tol and noticed that the option to clear "Cookies and other site data" has a description that states "Signs you out of most sites. You won't be signed out of your Google Account."
What he discovered was that if you were logged into Google and tried to clear all cookies, the Google authentications would not be removed, or if they were removed, were quickly recreated.
This same behavior occurs if you go into the list of all the cookies stored in Chrome and click on the Remove All button. When you click on this button you will see a message that states that all data stored on the device related to the shown sites would be deleted. Google.com is one of these listed sites.
After clicking on the Clear All button, though, and refreshing the Cookies screen, Google authentication cookies are restored.
According to Adrienne Porter Felt, an engineer & manager for the Google Chrome team, to permanently remove Google authentication cookies you first need to sign out of Google and then clear the cookies.
You can sign out, then clear those cookies.— Adrienne Porter Felt (@__apf__) September 25, 2018
Felt's answer, though, does not explain why this is happening. The general theory is that it is being caused by the new Chrome 69 behavior of being forcibly logged into the browser when you log into your Google account.
For those who want to see how Google cookies are not removed, or at least restored, when you clear all browsing data, you can view the video I created below.
Persistent authentication in Google since the release of Chrome 69 has been a cause of concern for users. While Google has been quick to respond and explain why these issues are occurring, they need to start giving control back to their users.
If a users deletes ALL of their cookies in Chrome, it should be expected and honored that the cookies belonging to Google will be removed as well.