CLN: Enforce deprecation of pinning name in SeriesGroupBy.agg (#57671)

* CLN: Enforce deprecation of pinning name in SeriesGroupBy.agg

* whatsnew

Author: rhshadrach

Diff for: doc/source/whatsnew/v3.0.0.rst

@@ -191,6 +191,7 @@ Removal of prior version deprecations/changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 - :func:`read_excel`, :func:`read_json`, :func:`read_html`, and :func:`read_xml` no longer accept raw string or byte representation of the data. That type of data must be wrapped in a :py:class:`StringIO` or :py:class:`BytesIO` (:issue:`53767`)
 - :meth:`Series.dt.to_pydatetime` now returns a :class:`Series` of :py:class:`datetime.datetime` objects (:issue:`52459`)
+- :meth:`SeriesGroupBy.agg` no longer pins the name of the group to the input passed to the provided ``func`` (:issue:`51703`)
 - All arguments except ``name`` in :meth:`Index.rename` are now keyword only (:issue:`56493`)
 - All arguments except the first ``path``-like argument in IO writers are now keyword only (:issue:`54229`)
 - All arguments in :meth:`Index.sort_values` are now keyword only (:issue:`56493`)
@@ -238,7 +239,6 @@ Removal of prior version deprecations/changes
 - Removed unused arguments ``*args`` and ``**kwargs`` in :class:`Resampler` methods (:issue:`50977`)
 - Unrecognized timezones when parsing strings to datetimes now raises a ``ValueError`` (:issue:`51477`)
 
-
 .. ---------------------------------------------------------------------------
 .. _whatsnew_300.performance:
 

Diff for: pandas/core/groupby/generic.py

@@ -62,10 +62,7 @@
 )
 import pandas.core.common as com
 from pandas.core.frame import DataFrame
-from pandas.core.groupby import (
-    base,
-    ops,
-)
+from pandas.core.groupby import base
 from pandas.core.groupby.groupby import (
     GroupBy,
     GroupByPlot,
@@ -373,32 +370,7 @@ def aggregate(self, func=None, *args, engine=None, engine_kwargs=None, **kwargs)
                     index=self._grouper.result_index,
                     dtype=obj.dtype,
                 )
-
-            if self._grouper.nkeys > 1:
-                return self._python_agg_general(func, *args, **kwargs)
-
-            try:
-                return self._python_agg_general(func, *args, **kwargs)
-            except KeyError:
-                # KeyError raised in test_groupby.test_basic is bc the func does
-                #  a dictionary lookup on group.name, but group name is not
-                #  pinned in _python_agg_general, only in _aggregate_named
-                result = self._aggregate_named(func, *args, **kwargs)
-
-                warnings.warn(
-                    "Pinning the groupby key to each group in "
-                    f"{type(self).__name__}.agg is deprecated, and cases that "
-                    "relied on it will raise in a future version. "
-                    "If your operation requires utilizing the groupby keys, "
-                    "iterate over the groupby object instead.",
-                    FutureWarning,
-                    stacklevel=find_stack_level(),
-                )
-
-                # result is a dict whose keys are the elements of result_index
-                result = Series(result, index=self._grouper.result_index)
-                result = self._wrap_aggregated_output(result)
-                return result
+            return self._python_agg_general(func, *args, **kwargs)
 
     agg = aggregate
 
@@ -527,26 +499,6 @@ def _wrap_applied_output(
                 result.index = default_index(len(result))
             return result
 
-    def _aggregate_named(self, func, *args, **kwargs):
-        # Note: this is very similar to _aggregate_series_pure_python,
-        #  but that does not pin group.name
-        result = {}
-        initialized = False
-
-        for name, group in self._grouper.get_iterator(self._obj_with_exclusions):
-            # needed for pandas/tests/groupby/test_groupby.py::test_basic_aggregations
-            object.__setattr__(group, "name", name)
-
-            output = func(group, *args, **kwargs)
-            output = ops.extract_result(output)
-            if not initialized:
-                # We only do this validation on the first iteration
-                ops.check_result_array(output, group.dtype)
-                initialized = True
-            result[name] = output
-
-        return result
-
     __examples_series_doc = dedent(
         """
     >>> ser = pd.Series([390.0, 350.0, 30.0, 20.0],

Diff for: pandas/tests/groupby/test_reductions.py

@@ -65,16 +65,6 @@ def test_basic_aggregations(dtype):
     with pytest.raises(pd.errors.SpecificationError, match=msg):
         grouped.aggregate({"one": np.mean, "two": np.std})
 
-    group_constants = {0: 10, 1: 20, 2: 30}
-    msg = (
-        "Pinning the groupby key to each group in SeriesGroupBy.agg is deprecated, "
-        "and cases that relied on it will raise in a future version"
-    )
-    with tm.assert_produces_warning(FutureWarning, match=msg):
-        # GH#41090
-        agged = grouped.agg(lambda x: group_constants[x.name] + x.mean())
-    assert agged[1] == 21
-
     # corner cases
     msg = "Must produce aggregated value"
     # exception raised is type Exception