Add whatsnew entry

Author: avinashpancham

doc/source/whatsnew/v1.2.0.rst

@@ -268,6 +268,7 @@ Deprecations
 - Deprecated :meth:`Index.is_all_dates` (:issue:`27744`)
 - Deprecated automatic alignment on comparison operations between :class:`DataFrame` and :class:`Series`, do ``frame, ser = frame.align(ser, axis=1, copy=False)`` before e.g. ``frame == ser`` (:issue:`28759`)
 - :meth:`Rolling.count` with ``min_periods=None`` will default to the size of the window in a future version (:issue:`31302`)
+- Deprecate use of strings denoting units with 'M', 'Y', 'm' and 'y' in :func:`~pandas.to_timedelta` (:issue:`36666`)
 
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