diff --git a/lisp/emacs-lisp/cl-lib.el b/lisp/emacs-lisp/cl-lib.el
index 7fee780a735..28d42cb69ab 100644
--- a/lisp/emacs-lisp/cl-lib.el
+++ b/lisp/emacs-lisp/cl-lib.el
@@ -560,6 +560,7 @@ cl-old-struct-compat-mode
     (advice-add 'type-of :around #'cl--old-struct-type-of))
    (t
     (advice-remove 'type-of #'cl--old-struct-type-of))))
+(make-obsolete 'cl-old-struct-compat-mode nil "30.1")
 
 (defun cl-constantly (value)
   "Return a function that takes any number of arguments, but returns VALUE."
diff --git a/lisp/emacs-lisp/cl-preloaded.el b/lisp/emacs-lisp/cl-preloaded.el
index 9445093f143..da2780503b8 100644
--- a/lisp/emacs-lisp/cl-preloaded.el
+++ b/lisp/emacs-lisp/cl-preloaded.el
@@ -158,7 +158,9 @@ cl-struct-define
   (cl-check-type name (satisfies cl--struct-name-p))
   (unless type
     ;; Legacy defstruct, using tagged vectors.  Enable backward compatibility.
-    (cl-old-struct-compat-mode 1))
+    (with-suppressed-warnings ((obsolete cl-old-struct-compat-mode))
+      (message "cl-old-struct-compat-mode is obsolete!")
+      (cl-old-struct-compat-mode 1)))
   (if (eq type 'record)
       ;; Defstruct using record objects.
       (setq type nil))