Ned, the warning is non-specific as to what might go wrong.  I would not want to remove it until I had tested everything possibly relevant with tabbing set.  And there is still the possibility that people will run IDLE with earlier tk releases.

The #33397 issue of the close button disappearing was solved by packing the close button first.  In the editor, the status bar is already packed into the ListedTopLevel first, before the text frame (perhaps for the same reason).