Can you push something like this to avoid a warning? I don't remember if I can commit.

--- a/swank/backend.lisp
+++ b/swank/backend.lisp
@@ -1386,6 +1386,9 @@ return nil.
 
 Return :interrupt if an interrupt occurs while waiting."
   (declare (ignore streams timeout))
+  ;; This defvar can't be moved to 'backend.lisp' because the initializing form isn't evaluable
+  ;; until an implementation method is loaded for (preferred-communication-style)
+  (declare (special swank:*communication-style*))
   ;; Invoking the slime debugger will just endlessly loop.
   (call-with-debugger-hook
    nil