Not sure how to test this. I read through T533 and found this command but it does not restrict Apt traffic.

sudo rm /var/run/sdwdate/* && sudo service sdwdate restart && sudo service tor restart && whonixcheck_tor_bootstrap_wait_max=10 whonixcheck --gui --cli

Also edited :

/etc/whonix_firewall.d/30_whonix_workstation_default.conf

Changed firewall_mode=full -> firewall_mode=timesync-fail-closed but Apt traffic still possible.

Obviously I'm not going in the right directions with this. Or doing something wrong?

Got it. Set firewall_mode=timesync-fail-closed in sys-whonix and reload whonix_firewall. When that is done both whonix-ws-14 and whonix-gw-14 upgrades fail.

Ign:1 tor+http://sgvtcaew4bxjd7ln.onion stretch/updates InRelease
Ign:2 http://vwakviie2ienjx6t.onion/debian stretch-backports InRelease                    
Err:3 tor+http://sgvtcaew4bxjd7ln.onion stretch/updates Release                           
  500  Unable to connect
[...]
E: The repository 'tor+http://sgvtcaew4bxjd7ln.onion stretch/updates Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.