setup method on registered blueprint is error

CHANGES.rst

@@ -20,6 +20,8 @@ Unreleased
     -   ``json_encoder`` and ``json_decoder`` attributes on app and blueprint, and the
         corresponding ``json.JSONEncoder`` and ``JSONDecoder`` classes, are removed.
     -   The ``json.htmlsafe_dumps`` and ``htmlsafe_dump`` functions are removed.
+    -   Calling setup methods on blueprints after registration is an error instead of a
+        warning. :pr:`4997`
 
 -   Importing ``escape`` and ``Markup`` from ``flask`` is deprecated. Import them
     directly from ``markupsafe`` instead. :pr:`4996`

src/flask/blueprints.py

@@ -207,19 +207,12 @@ class Blueprint(Scaffold):
 
     def _check_setup_finished(self, f_name: str) -> None:
         if self._got_registered_once:
-            import warnings
-
-            warnings.warn(
-                f"The setup method '{f_name}' can no longer be called on"
-                f" the blueprint '{self.name}'. It has already been"
-                " registered at least once, any changes will not be"
-                " applied consistently.\n"
-                "Make sure all imports, decorators, functions, etc."
-                " needed to set up the blueprint are done before"
-                " registering it.\n"
-                "This warning will become an exception in Flask 2.3.",
-                UserWarning,
-                stacklevel=3,
+            raise AssertionError(
+                f"The setup method '{f_name}' can no longer be called on the blueprint"
+                f" '{self.name}'. It has already been registered at least once, any"
+                " changes will not be applied consistently.\n"
+                "Make sure all imports, decorators, functions, etc. needed to set up"
+                " the blueprint are done before registering it."
             )
 
     @setupmethod