GNU Guile 3.0.1
Copyright (C) 1995-2020 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> (define str2 "hello")
scheme@(guile-user)> (string-set! str2 4 #\a)
ice-9/boot-9.scm:1669:16: In procedure raise-exception:
string is read-only: "hello"

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [1]> ,q
scheme@(guile-user)> (string? str2)
#t

is it a bug in Guile ? :-O

i can only find reference to deprecated read-only string in old doc:
https://www.gnu.org/software/guile/docs/docs-1.6/guile-ref/Read-Only-Strings.html#Read%20Only%20Strings

Regards,

Damien