I think a tree view was promised (listed on a presentation) when Flutter 2.0 was released, but no details were provided, and I haven't heard of it ever since.
You might be thinking of this package, authored by Google, mostly by people on the Flutter team but it's not an official Flutter project (as far as I know): https://pub.dev/packages/flutter_simple_treeview
(Disclaimer: I work on the Flutter team.)