diff --git a/doc/api/next_api_changes/2019-02-21-AL.rst b/doc/api/next_api_changes/2019-02-21-AL.rst
new file mode 100644
index 000000000000..dc12a363ead6
--- /dev/null
+++ b/doc/api/next_api_changes/2019-02-21-AL.rst
@@ -0,0 +1,6 @@
+Deprecations
+````````````
+
+The ``backend_wx.IDLE_DELAY`` global, being unused and only relevant to the now
+removed wx "idling" code, is deprecated (note that as it is a module-level
+global, no deprecation warning is emitted when accessing it).
diff --git a/lib/matplotlib/backends/backend_wx.py b/lib/matplotlib/backends/backend_wx.py
index 1a86b28ad98b..01ce0061106f 100644
--- a/lib/matplotlib/backends/backend_wx.py
+++ b/lib/matplotlib/backends/backend_wx.py
@@ -75,7 +75,7 @@ def write(self, msg):
 PIXELS_PER_INCH = 75
 
 # Delay time for idle checks
-IDLE_DELAY = 5
+IDLE_DELAY = 5  # Documented as deprecated as of Matplotlib 3.1.
 
 
 def error_msg_wx(msg, parent=None):