AFAIK, the Windows x.y compiled release and installer is normally updated to the latest stable tcl/tk, about the time of beta 1, and never changed for bugfix releases.  2.7 was an exception because of its long life and because later 8.5.z releases has some critical bugfixes, such as working with European keyboards.

I believe the current OSX binary compiles _tkinter against tcl/tk 8.5, and we recommend replacing the Apple's old 8.5 with the latest 8.5 from ActiveState, and that people who instead install ActiveState 8.6 have to make or obtain a new Python binary that compiles _tkinter for 8.6. (Prior to Py3.6, there was also a Mac release for older Macs, and it was compiled for tcl/tk 8.4.)

We don't compile Python on *nix and there are multiple versions of tcl/tk in use.  I would prefer to have the latest tcl/tk at least for bugfixes, and a Python binary that can use if, even if tkinter does not give access to the latest tk features.