A partial workaround to the problem: for Mail.app and Notes.app you can add your Google account as a generic "Mail Account". For this to work you'll need to generate an application-specific password in your Google account and use this app-specific password with your Google email address to link the account to Mail.app and macOS.
Re-download the 10.14.6 MacOS from the apple store and reinstall it.
You only need to wait for about 30 minutes.
After reinstalling, don't update Safari from the Apple Store for now.
After I waited for 3 days, this is the fastest solution...
Google Calendar has no fix as of yet, You could try CalDAV but it no longer works, for Contacts add a CardDAV account, set the Account Type to Manual.
Server Address: www.google.com
User name: Your email address
Password: Your google password
You might get an unable to connect to account error, to fix that, You need to enable Low Security App access.
As of the time this was posted there's no known solution to the problem. Safari 13.0 broke the authentication workflow from System Preferences with Google account authentication if you use 2FA on your Google account.
The problem has been reported to Google and Apple via their official bug feedback channels.
A partial workaround to the ...
If, like me, you have that problem, you might want to check that googleapis.com and other google domains aren't blocked by a firewall (if you are using little snitch, double check that accountsd can access google apis.
Once I allowed accountsd outgoing internet access to googleapis.com, as shown in the attached screenshot, the mac no longer complained when ...