Refs #31369 -- Deprecated models.NullBooleanField in favor of BooleanField(null=True).

docs/internals/deprecation.txt

@@ -75,6 +75,9 @@ details on these changes.
 * Support for passing raw column aliases to ``QuerySet.order_by()`` will be
   removed.
 
+* The model ``NullBooleanField`` will be removed. A stub field will remain for
+  compatibility with historical migrations.
+
 See the :ref:`Django 3.1 release notes <deprecated-features-3.1>` for more
 details on these changes.
 

docs/ref/checks.txt

@@ -202,6 +202,8 @@ Model fields
   in historical migrations.
 * **fields.W902**: ``FloatRangeField`` is deprecated and will be removed in
   Django 3.1. *This check appeared in Django 2.2 and 3.0*.
+* **fields.W903**: ``NullBooleanField`` is deprecated. Support for it (except
+  in historical migrations) will be removed in Django 4.0.
 
 File fields
 ~~~~~~~~~~~

docs/ref/models/fields.txt

@@ -1182,8 +1182,11 @@ values are stored as null.
 
 .. class:: NullBooleanField(**options)
 
-Like :class:`BooleanField` with ``null=True``. Use that instead of this field
-as it's likely to be deprecated in a future version of Django.
+Like :class:`BooleanField` with ``null=True``.
+
+.. deprecated:: 3.1
+
+    ``NullBooleanField`` is deprecated in favor of ``BooleanField(null=True)``.
 
 ``PositiveBigIntegerField``
 ---------------------------

docs/releases/3.1.txt

@@ -733,6 +733,9 @@ Miscellaneous
   same result can be achieved by passing aliases in a
   :class:`~django.db.models.expressions.RawSQL` instead beforehand.
 
+* The ``NullBooleanField`` model field is deprecated in favor of
+  ``BooleanField(null=True)``.
+
 .. _removed-features-3.1:
 
 Features removed in 3.1