Refs #31369 -- Removed models.NullBooleanField per deprecation timeline.

docs/ref/checks.txt

@@ -206,7 +206,10 @@ Model fields
 * **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.
+  in historical migrations) will be removed in Django 4.0. *This check appeared
+  in Django 3.1 and 3.2*.
+* **fields.E903**: ``NullBooleanField`` is removed except for support in
+  historical migrations.
 * **fields.W904**: ``django.contrib.postgres.fields.JSONField`` is deprecated.
   Support for it (except in historical migrations) will be removed in Django
   4.0.

docs/ref/models/fields.txt

@@ -1254,17 +1254,6 @@ To query ``JSONField`` in the database, see :ref:`querying-jsonfield`.
     objects and arrays (represented in Python using :py:class:`dict` and
     :py:class:`list`) are supported.
 
-``NullBooleanField``
---------------------
-
-.. class:: NullBooleanField(**options)
-
-Like :class:`BooleanField` with ``null=True``.
-
-.. deprecated:: 3.1
-
-    ``NullBooleanField`` is deprecated in favor of ``BooleanField(null=True)``.
-
 ``PositiveBigIntegerField``
 ---------------------------
 

docs/releases/2.1.txt

@@ -189,8 +189,8 @@ Models
   now support using field transforms.
 
 * :class:`~django.db.models.BooleanField` can now be ``null=True``. This is
-  encouraged instead of :class:`~django.db.models.NullBooleanField`, which will
-  likely be deprecated in the future.
+  encouraged instead of ``NullBooleanField``, which will likely be deprecated
+  in the future.
 
 * The new :meth:`.QuerySet.explain` method displays the database's execution
   plan of a queryset's query.

docs/releases/4.0.txt

@@ -305,3 +305,6 @@ to remove usage of these features.
 * The ``list`` message for ``ModelMultipleChoiceField`` is removed.
 
 * Support for passing raw column aliases to ``QuerySet.order_by()`` is removed.
+
+* The ``NullBooleanField`` model field is removed, except for support in
+  historical migrations.

docs/topics/forms/modelforms.txt

@@ -106,8 +106,6 @@ Model field                         Form field
 :class:`ManyToManyField`            :class:`~django.forms.ModelMultipleChoiceField`
                                     (see below)
 
-:class:`NullBooleanField`           :class:`~django.forms.NullBooleanField`
-
 :class:`PositiveBigIntegerField`    :class:`~django.forms.IntegerField`
 
 :class:`PositiveIntegerField`       :class:`~django.forms.IntegerField`