The following test from the REPL was used to determine which Schemes, if any, return immutable pairs from calling read on the input (a . b):
(define x (read)) (a . b) (set-car! x 'b) xIronScheme, S7, Dfsch, Inlab don't support (read) direct from the command line, so I substituted reading from a file that contained nothing but (a . b).
Output (b . b) per R![57]RS: Gauche, MIT, Gambit, Chicken, Bigloo, Scheme48/scsh, Guile, Kawa, SISC, Detroit, SCM, Chez, Vicare, Larceny, Ypsilon, Mosh, IronScheme, JScheme, STklos, KSi, SigScheme, MiniScheme, TinyScheme, S7, Scheme 9, RScheme, Unlikely, SIOD, BDC, XLisp, Elk, SXM, Sizzle, FemtoLisp, Inlab, Sagittarius, Foment, Picrin, Chibi
All pairs are immutable: Racket, Owl Lisp
Returned pair is immutable: JScheme
No usable read, set-car! or open-input-file: Rep, Shoe, UMB, Llava, Dfsch