DEPR: change Panel DeprecationWarning -> FutureWarning (#19247)

closes #18324

Author: jreback

doc/source/whatsnew/v0.23.0.txt

@@ -225,6 +225,34 @@ If installed, we now require:
    | openpyxl        | 2.4.0           |          |
    +-----------------+-----------------+----------+
 
+.. _whatsnew_0230.api_breaking.deprecate_panel:
+
+Deprecate Panel
+^^^^^^^^^^^^^^^
+
+``Panel`` was deprecated in the 0.20.x release, showing as a ``DeprecationWarning``. Using ``Panel`` will now show a ``FutureWarning``. The recommended way to represent 3-D data are
+with a ``MultiIndex`` on a ``DataFrame`` via the :meth:`~Panel.to_frame` or with the `xarray package <http://xarray.pydata.org/en/stable/>`__. Pandas
+provides a :meth:`~Panel.to_xarray` method to automate this conversion. For more details see :ref:`Deprecate Panel <dsintro.deprecate_panel>` documentation. (:issue:`13563`, :issue:`18324`).
+
+.. ipython:: python
+   :okwarning:
+
+   p = tm.makePanel()
+   p
+
+Convert to a MultiIndex DataFrame
+
+.. ipython:: python
+
+   p.to_frame()
+
+Convert to an xarray DataArray
+
+.. ipython:: python
+   :okwarning:
+
+   p.to_xarray()
+
 
 Build Changes
 ^^^^^^^^^^^^^
@@ -291,6 +319,7 @@ Deprecations
 - :func:`read_excel` has deprecated the ``skip_footer`` parameter. Use ``skipfooter`` instead (:issue:`18836`)
 - The ``is_copy`` attribute is deprecated and will be removed in a future version (:issue:`18801`).
 
+
 .. _whatsnew_0230.prior_deprecations:
 
 Removal of prior version deprecations/changes

pandas/core/panel.py

@@ -151,7 +151,7 @@ def __init__(self, data=None, items=None, major_axis=None, minor_axis=None,
                       "http://xarray.pydata.org/en/stable/.\n"
                       "Pandas provides a `.to_xarray()` method to help "
                       "automate this conversion.\n",
-                      DeprecationWarning, stacklevel=3)
+                      FutureWarning, stacklevel=3)
 
         self._init_data(data=data, items=items, major_axis=major_axis,
                         minor_axis=minor_axis, copy=copy, dtype=dtype)

pandas/tests/generic/test_label_or_level_utils.py

@@ -46,7 +46,7 @@ def df_duplabels(df):
 
 @pytest.fixture
 def panel():
-    with tm.assert_produces_warning(DeprecationWarning,
+    with tm.assert_produces_warning(FutureWarning,
                                     check_stacklevel=False):
         return pd.Panel()