You can fake it in a query at the moment but it is ugly as hell and doesn't cope with the last child, showing you a No Records Query Result (and I haven't been able to put operations on child records and display a form in the default output adequately - the sub queries are in their own layout inside the main query layout.)
Faked Hierarchy
Screen Shot 2021-05-21 at 15.58.24.png (30.48 KiB) Viewed 29513 times
Or you can fake it in a form, but you can't get the root BO into the hierarchy (if you include OB and OM attributes into the one tree you get an infinite recursive loop), and the tree in the form cannot be set to full screen - it must have a defined height.
Form Hierarchy
Screen Shot 2021-05-21 at 16.00.00.png (39.31 KiB) Viewed 29513 times