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

django/db/models/fields/__init__.py

@@ -1929,6 +1929,14 @@ class NullBooleanField(BooleanField):
         'invalid_nullable': _('“%(value)s” value must be either None, True or False.'),
     }
     description = _("Boolean (Either True, False or None)")
+    system_check_deprecated_details = {
+        'msg': (
+            'NullBooleanField is deprecated. Support for it (except in '
+            'historical migrations) will be removed in Django 4.0.'
+        ),
+        'hint': 'Use BooleanField(null=True) instead.',
+        'id': 'fields.W903',
+    }
 
     def __init__(self, *args, **kwargs):
         kwargs['null'] = True