deprecate before_first_request

src/flask/blueprints.py

@@ -543,7 +543,20 @@ class Blueprint(Scaffold):
     ) -> ft.BeforeFirstRequestCallable:
         """Like :meth:`Flask.before_first_request`.  Such a function is
         executed before the first request to the application.
+
+        .. deprecated:: 2.2
+            Will be removed in Flask 2.3. Run setup code when creating
+            the application instead.
         """
+        import warnings
+
+        warnings.warn(
+            "'before_app_first_request' is deprecated and will be"
+            " removed in Flask 2.3. Use 'record_once' instead to run"
+            " setup code when registering the blueprint.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
         self.record_once(lambda s: s.app.before_first_request_funcs.append(f))
         return f