One of our faculty noticed that surveys in Canvas can't be officially anonymous. Students need to know that their feedback is actually anonymous for surveys to work. Right now, an anonymous survey can be made anonymous and then edited after the students have responded to make it no longer anonymous. This allows the teacher to see the student individual responses. A simple solution would be to grey out that button once students have started taking the quiz/survey just like some of the other features in the quiz/survey once the survey has been started.
I found a similar request in the archives: