We are trying to add a new tier to Fiddler's update functionality that will allow the user to be notified about critical/emergency updates even if the auto update is disabled. This is what you saw happening in Fiddler 4.6.2.3. Of course, that too can be disabled but it is not controlled by the "Check for updates on startup" setting.

So if the "Check for updates on startup" check-box is empty Fiddler will contact the update service but you will not get update notification unless there is critical update available. Since critical updates are pretty rare the user experience should not really change.

The existing noversioncheck command line option and BlockUpdateCheck registry setting will disable the critical updates check as well as the regular updates check. This comes to preserve the user experience for the users already making use of these options. It also serves as a way to disable the critical update check should one wish to do so.

We were debating on using a different URL for the critical updates check to avoid confusion but the chances are the auto update URL is already part of a large number of exception/rules lists so adding a new one would have resulted in a lot of hassle.

Last but not least, thank you very much for reporting this. Apparently we need to do a better job explaining what is going on with that surprising update call in the UI of Fiddler.