It's a 401 issue because it's a 401 not a 404 or any other things

Done here : https://github.com/LimeSurvey/LimeSurvey/blob/016b4220056ef12c52d54e64c8ca11a5b359b45f/application/controllers/survey/index.php#L91-L94

My opinion : muts be always a 404 if survey are noot accessible. No difference betwwen exiting of not existing survey.

It's a Point Of View issue, not a real issue.

What I mean is: you can say it is a 401 or anything, but Limesurvey probably knows what the reason for the 401 is, so it could give additional information (e.g. "This survey is currently not active"). Would be a lot more user friendly. Not everyone is a developer, especially not the respondents.

My opinion : must be always a 404 if survey are not accessible. No difference between exiting of not existing survey.

Let's ask some users what they would prefer... ;-)