Warn on unrecognized Configurable kwargs with UserWarning

tests/test_traitlets.py

@@ -2829,6 +2829,25 @@ class SuperHasTraits(HasTraits):
     assert not hasattr(obj, "b")
 
 
+def test_configurable_unrecognized_args_user_warning():
+    """Configurable should surface unknown kwargs as UserWarning (#926)."""
+    import warnings
+
+    from traitlets.config.configurable import Configurable
+
+    with warnings.catch_warnings(record=True) as caught:
+        warnings.simplefilter("always")
+        Configurable(not_a_trait=1)
+    user = [w for w in caught if issubclass(w.category, UserWarning)]
+    assert user
+    assert "Passing unrecognized arguments" in str(user[0].message)
+    assert not any(
+        issubclass(w.category, DeprecationWarning)
+        and "Passing unrecognized arguments" in str(w.message)
+        for w in caught
+    )
+
+
 def test_default_mro():
     """Verify that default values follow mro"""
 

traitlets/traitlets.py

@@ -1399,7 +1399,7 @@ def ignore(change: Bunch) -> None:
                 f"{e}\n"
                 "This is deprecated in traitlets 4.2."
                 "This error will be raised in a future release of traitlets.",
-                DeprecationWarning,
+                UserWarning,
                 stacklevel=2,
             )