In ac.scm, in the arc compiler ac, arc strings are translated using ac-string. I've tried instrumenting the output of ac-string, and it seems mutable when it's encountered. And yet it's immutable by the time I return to the prompt:
I tried bisecting the git history, and the stable branch, which still requires mzscheme, has mutable strings. But sometime in 2011 when the master branch switched to using racket and stopped working with mzscheme, strings went immutable again. Still investigating..
Update 34 minutes later: It seems eval in racket emits immutable strings even when given mutable strings. Since everything passes through racket's eval it doesn't matter how much arc twists and turns to avoid immutability.