I found tix8.4.3.dll and pkgIndex.tcl inside tcl/tix8.4.3, which is not where the doc currently says to look.

Christian Gollwitzer posted a response on the python-list thread "Python Tk Tix GUI documentation & builder overview and tips" (which helped inspire this issue in addition to the SO post).  He basically agrees with Ned's ending comment that "we should be strongly discouraging use of Tix in favor of Ttk."

"Note that Tix is LEGACY. There are much better options for its widgets available that are based on ttk. ... Not to mention that the Tix widgets are extremely ugly, while the widgets from ttk have a near-native look and feel."

So I agree that the opening paragraph of the (T)tkinter.tix section 
https://docs.python.org/3/library/tkinter.tix.html#module-tkinter.tix
should identify it as a legacy python interface to the old, optional, tcl/tk tix extension that has been mostly superceded by widgets added either directly to tk or by ttk.

Cristian also listed the ttk replacements.
'''
* Hierarchical Listbox use ttk::treeview, tablelist_tile or tkTreeCtrl, with increasing order of capabilities and install complexity

* TList: I think tkTreeCtrl can do it, never have used this layout

* CheckList: tablelist can be used with an editable field

* Grid widget: use tkTable

* NoteBook, PanedWindow, ComboBox, LabelFrame -> ttk::notebook, ttk::panedwindow, ttk::combobox, ttk::labelframe

* Balloon: tcllib::tooltip
'''

The tkinter and tkinter.ttk replacements could similarly be added to the tix widget listing in the doc.