At the moment a survey link for an open survey has 6 digits, the survey ID. Would it be interesting the make this number more complex, longer and less easy to guess. For example fdT58630310A6dN91a7d8F0a4ceda2. This way the survey is still open, but de facto only accessible by people who have the code.

I don't see a way to change the surveyid itself. It's needed for in many ways.

You might have a feature which would allow to change the URL to access the survey and block the URL via the surveyID.

Most feature request want to get rid of the ID and add easy to remember text.

bugs.limesurvey.org/view.php?id=18234