Added M and Y units deprecated test and whats new note

Author: ryankarlos

doc/source/whatsnew/v0.24.0.txt

@@ -972,6 +972,7 @@ Deprecations
   `use_threads` to reflect the changes in pyarrow 0.11.0. (:issue:`23053`)
 - :func:`pandas.read_excel` has deprecated accepting ``usecols`` as an integer. Please pass in a list of ints from 0 to ``usecols`` inclusive instead (:issue:`23527`)
 - Constructing a :class:`TimedeltaIndex` from data with ``datetime64``-dtyped data is deprecated, will raise ``TypeError`` in a future version (:issue:`23539`)
+- Deprecated the `M` and `Y` `units` parameter of :func: `pandas.to_timedelta`, :func: `pandas.Timedelta` and :func: `pandas.TimedeltaIndex`
 
 .. _whatsnew_0240.deprecations.datetimelike_int_ops:
 

pandas/core/tools/timedeltas.py

@@ -80,6 +80,10 @@ def to_timedelta(arg, unit='ns', box=True, errors='raise'):
         raise ValueError("errors must be one of 'ignore', "
                          "'raise', or 'coerce'}")
 
+    if unit in ['Y', 'y', 'M']:
+        warnings.warn("M and Y units are deprecated.",
+                      FutureWarning, stacklevel=2)
+
     if arg is None:
         return arg
     elif isinstance(arg, ABCSeries):

pandas/tests/scalar/timedelta/test_timedelta.py

@@ -352,6 +352,14 @@ def test_unit_parser(self, units, np_unit, wrapper):
             result = Timedelta('2{}'.format(unit))
             assert result == expected
 
+    @pytest.mark.parametrize('units', ['Y', 'y', 'M'])
+    def test_unit_M_Y_deprecated(self, units):
+        for unit in units:
+            with tm.assert_produces_warning(FutureWarning):
+                to_timedelta(10, unit)
+                TimedeltaIndex([1, 1, 1], unit)
+                Timedelta(10, unit)
+
     def test_numeric_conversions(self):
         assert ct(0) == np.timedelta64(0, 'ns')
         assert ct(10) == np.timedelta64(10, 'ns')