diff --git a/doc/api/next_api_changes/behavior/20268-JMK.rst b/doc/api/next_api_changes/behavior/20268-JMK.rst
new file mode 100644
index 000000000000..e0ff20bfcc82
--- /dev/null
+++ b/doc/api/next_api_changes/behavior/20268-JMK.rst
@@ -0,0 +1,11 @@
+pcolor(mesh) shading defaults to auto
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The *shading* kwarg for `.Axes.pcolormesh` and `.Axes.pcolor` default 
+has been changed to 'auto'.  
+
+Passing ``Z(M, N)``, ``x(N)``, ``y(M)`` to ``pcolormesh`` with 
+``shading='flat'`` will now raise a ``TypeError``.  Use 
+``shading='auto'`` or ``shading='nearest'`` for ``x`` and ``y``
+to be treated as cell centers, or drop the last column and row 
+of ``Z`` to get the old behavior with ``shading='flat'``.      
\ No newline at end of file
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
index ee4c2b5e371b..58994cc8c456 100644
--- a/lib/matplotlib/axes/_axes.py
+++ b/lib/matplotlib/axes/_axes.py
@@ -5603,7 +5603,7 @@ def imshow(self, X, cmap=None, norm=None, aspect=None,
         self.add_image(im)
         return im
 
-    def _pcolorargs(self, funcname, *args, shading='flat', **kwargs):
+    def _pcolorargs(self, funcname, *args, shading='auto', **kwargs):
         # - create X and Y if not present;
         # - reshape X and Y as needed if they are 1-D;
         # - check for proper sizes based on `shading` kwarg;
@@ -5675,25 +5675,16 @@ def _pcolorargs(self, funcname, *args, shading='flat', **kwargs):
                 shading = 'flat'
 
         if shading == 'flat':
-            if not (ncols in (Nx, Nx - 1) and nrows in (Ny, Ny - 1)):
+            if (Nx, Ny) != (ncols + 1, nrows + 1):
                 raise TypeError('Dimensions of C %s are incompatible with'
                                 ' X (%d) and/or Y (%d); see help(%s)' % (
                                     C.shape, Nx, Ny, funcname))
-            if (ncols == Nx or nrows == Ny):
-                _api.warn_deprecated(
-                    "3.3", message="shading='flat' when X and Y have the same "
-                    "dimensions as C is deprecated since %(since)s.  Either "
-                    "specify the corners of the quadrilaterals with X and Y, "
-                    "or pass shading='auto', 'nearest' or 'gouraud', or set "
-                    "rcParams['pcolor.shading'].  This will become an error "
-                    "%(removal)s.")
-            C = C[:Ny - 1, :Nx - 1]
         else:    # ['nearest', 'gouraud']:
             if (Nx, Ny) != (ncols, nrows):
                 raise TypeError('Dimensions of C %s are incompatible with'
                                 ' X (%d) and/or Y (%d); see help(%s)' % (
                                     C.shape, Nx, Ny, funcname))
-            if shading in ['nearest', 'auto']:
+            if shading == 'nearest':
                 # grid is specified at the center, so define corners
                 # at the midpoints between the grid centers and then use the
                 # flat algorithm.
diff --git a/lib/matplotlib/mpl-data/matplotlibrc b/lib/matplotlib/mpl-data/matplotlibrc
index 326d02c2124b..0b8cb1c86f35 100644
--- a/lib/matplotlib/mpl-data/matplotlibrc
+++ b/lib/matplotlib/mpl-data/matplotlibrc
@@ -130,7 +130,7 @@
 
 #markers.fillstyle: full  # {full, left, right, bottom, top, none}
 
-#pcolor.shading: flat
+#pcolor.shading: auto
 #pcolormesh.snap: True  # Whether to snap the mesh to pixel boundaries. This is
                         # provided solely to allow old test images to remain
                         # unchanged. Set to False to obtain the previous behavior.
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
index feb72f884fc6..3736888fdde5 100644
--- a/lib/matplotlib/tests/test_axes.py
+++ b/lib/matplotlib/tests/test_axes.py
@@ -1243,22 +1243,14 @@ def test_pcolornearestunits(fig_test, fig_ref):
     ax.pcolormesh(x2, y2, Z, shading='nearest')
 
 
-@check_figures_equal(extensions=["png"])
-def test_pcolordropdata(fig_test, fig_ref):
-    ax = fig_test.subplots()
-    x = np.arange(0, 10)
-    y = np.arange(0, 4)
+def test_pcolorflaterror():
+    fig, ax = plt.subplots()
+    x = np.arange(0, 9)
+    y = np.arange(0, 3)
     np.random.seed(19680801)
     Z = np.random.randn(3, 9)
-    # fake dropping the data
-    ax.pcolormesh(x[:-1], y[:-1], Z[:-1, :-1], shading='flat')
-
-    ax = fig_ref.subplots()
-    # test dropping the data...
-    x2 = x[:-1]
-    y2 = y[:-1]
-    with pytest.warns(MatplotlibDeprecationWarning):
-        ax.pcolormesh(x2, y2, Z, shading='flat')
+    with pytest.raises(TypeError, match='Dimensions of C'):
+        ax.pcolormesh(x, y, Z, shading='flat')
 
 
 @check_figures_equal(extensions=["png"])
@@ -1268,13 +1260,15 @@ def test_pcolorauto(fig_test, fig_ref):
     y = np.arange(0, 4)
     np.random.seed(19680801)
     Z = np.random.randn(3, 9)
-    ax.pcolormesh(x, y, Z, shading='auto')
+    # this is the same as flat; note that auto is default
+    ax.pcolormesh(x, y, Z)
 
     ax = fig_ref.subplots()
     # specify the centers
     x2 = x[:-1] + np.diff(x) / 2
     y2 = y[:-1] + np.diff(y) / 2
-    ax.pcolormesh(x2, y2, Z, shading='auto')
+    # this is same as nearest:
+    ax.pcolormesh(x2, y2, Z)
 
 
 @image_comparison(['canonical'])