NullBooleanField is deprecated and will be removed a future version of Django.
NullBooleanField is like BooleanField with null=True.
Historically the only values BooleanField accepted was True and False. BooleanField didn't permit null=True. If null was a valid value then NullBooleanField was used instead.
If we spot this issue in your GitHub pull request we give this advice:
Django Doctor will run this check by default. No configuration is needed but the check can be turned on/off using check code
deprecated-nullboolean-field in your pyproject.toml file.