`strict_mode` removed to improve forward compatibility.

setuptools/config.py

@@ -100,12 +100,6 @@ class ConfigHandler(object):
 
     """
 
-    strict_mode = True
-    """Flag. Whether unknown options in config should
-    raise DistutilsOptionError exception, or pass silently.
-
-    """
-
     def __init__(self, target_obj, options):
         sections = {}
 
@@ -316,9 +310,7 @@ class ConfigHandler(object):
                 self[name] = value
 
             except KeyError:
-                if self.strict_mode:
-                    raise DistutilsOptionError(
-                        'Unknown distribution option: %s' % name)
+                pass  # Keep silent for a new option may appear anytime.
 
     def parse(self):
         """Parses configuration file items from one

setuptools/tests/test_config.py

@@ -417,18 +417,6 @@ class TestOptions:
         with get_dist(tmpdir) as dist:
             assert dist.packages == ['fake_package']
 
-    def test_unknown_options_item(self, tmpdir):
-
-        fake_env(
-            tmpdir,
-            '[options]\n'
-            'zip_safe = True\n'
-            'usr_2to3 = 1\n'
-        )
-        with get_dist(tmpdir, parse=False) as dist:
-            with pytest.raises(DistutilsOptionError):
-                dist.parse_config_files()
-
     def test_extras_require(self, tmpdir):
         fake_env(
             tmpdir,