class NavigationDestination extends StatelessWidget {
...

  /// The text label that appears below the icon of this
  /// [NavigationDestination].
  ///
  /// The accompanying [Text] widget will use
  /// [NavigationBarThemeData.labelTextStyle]. If this are null, the default
  /// text style would use [TextTheme.labelSmall] with [ColorScheme.onSurface].
  final String label;

...
}

class _NavigationBarDefaultsM3 extends NavigationBarThemeData {
...

@override
  MaterialStateProperty<TextStyle?>? get labelTextStyle {
    return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
    final TextStyle style = _textTheme.labelMedium!;
      return style.apply(
        color: states.contains(MaterialState.disabled)
          ? _colors.onSurfaceVariant.withOpacity(0.38)
          : states.contains(MaterialState.selected)
            ? _colors.onSurface
            : _colors.onSurfaceVariant
      );
    });
  }

...
}