File: Doc/whatsnew/3.6.rst
Location within file: Under 'Improved Modules', as 'idlelib (IDLE)' (?)
Draft test:
"Idlelib is being revamped to make IDLE look better and work better and to make it easier to maintain and improve IDLE. Part of making IDLE look better, especially on Linux and Mac, is using ttk widgets.  As a result, IDLE no longer runs with tcl/tk 8.4.  It now requires tcl/tk 8.5 or 8.6 and availability of ttk widgets.  We recommend running the latest release of either.

'Revamping' includes renaming, refactoring, and consolidation of idlelib modules. As a result, imports of idlelib files that worked in 3.5 will usually not work in 3.6.  At least a module name change will be needed, sometimes more.  Information will be added to idlelib.  In compensation, the eventual result with be that some idlelib classes will be easier to use, with better APIs and docstrings explaining them."

This should be added before 3.6.0a2 and considered critical for that release, but after patches that make it true. #24759, using a revision of Serhiy's code from #24750, will make 3.5 and successful loading of ttk a requirement for IDLE (first paragraph).  New issue #27162 will add compatibility info and a new module (second paragraph) and require that the second paragraph be re-written a bit.