This minimal example violates the premise that deletion notes are generated only in relation to a form that the literally appeared in user-supplied source.

% cat /tmp/foo.lisp
(defun foo (x &optional (y (error "You had best give me Y"))) (+ x y))

COMMON-LISP-USER> (compile-file "/tmp/foo")
; compiling file "/private/tmp/foo.lisp" (written 25 JUL 2013 02:38:43 PM):
; compiling (DEFUN FOO ...)
; file: /private/tmp/foo.lisp
; in: DEFUN FOO
;     (DEFUN FOO (X &OPTIONAL (Y (ERROR "You had best give me Y"))) (+ X Y))
; --> PROGN EVAL-WHEN SB-IMPL::%DEFUN LET SB-C::%FUNCALL
; ==>
;   X
;
; note: deleting unreachable code
;
; compilation unit finished
;   printed 1 note

; /tmp/foo.fasl written
; compilation finished in 0:00:00.008
#P"/private/tmp/foo.fasl"

The note pertains to X because the Y's defaulting form signals an error.
Indeed one use of X is dead due to the way that optional dispatch works.
MAKE-XEP-LAMBDA-EXPRESSION ends up doing the equivalent of
   (cond ((eql n-args 1)
             (%funcall #<entry-point> X (form-that-never-returns)))
but the user didn't write that particular use of X, so it shouldn't be the subject of a complaint.

I doubt that this kind of defaulting form occurs often, however FORMATTER generates exactly that. Due to QPX's somewhat perverse use of FORMATTER we see the notes. (It "should" be invisible due to not having literally written the entire formatter expression as source, except that we *do* literally write it as source through explicit use of macroexpand on formatter).

Seems the general issue could be addressed though, since the deletion of X in the initial example is otherwise mystifying. Example with FORMATTER:

(macroexpand-1 '(formatter "Hi ~D ~D")) ->
#'(LAMBDA
      (STREAM
       &OPTIONAL
       (#:FORMAT-ARG613
        (ERROR 'SB-FORMAT:FORMAT-ERROR :COMPLAINT "required argument missing" :CONTROL-STRING
               "Hi ~D ~D" :OFFSET 4))
       (#:FORMAT-ARG614
        (ERROR 'SB-FORMAT:FORMAT-ERROR :COMPLAINT "required argument missing" :CONTROL-STRING
               "Hi ~D ~D" :OFFSET 7))
       &REST SB-FORMAT::ARGS)
    (DECLARE (IGNORABLE STREAM))
    (BLOCK NIL
      (WRITE-STRING "Hi " STREAM)
      (LET ((*PRINT-BASE* 10) (*PRINT-RADIX* NIL) (*PRINT-ESCAPE* NIL))
        (SB-KERNEL:OUTPUT-OBJECT #:FORMAT-ARG613 STREAM))
      (WRITE-STRING " " STREAM)
      (LET ((*PRINT-BASE* 10) (*PRINT-RADIX* NIL) (*PRINT-ESCAPE* NIL))
        (SB-KERNEL:OUTPUT-OBJECT #:FORMAT-ARG614 STREAM)))
    SB-FORMAT::ARGS)