A second specific reason to make ttk optional is either they or the accompanying re-writing may (and probably will) break some extension that goes beyond the narrowly defined extension interface.  For the present, a user using such an extension would be able to continue to do so, by turning use_ttk off. (I plan to add some DeprecationWarnings, either at startup or old tk module import, when use_ttk is possible but turned off.)

The normal bugfix-only policy, and the Idle exemption, starts with each x.y.0b1 release. The point of excluding 'major re-writes such as switching to themed widgets' was to exclude changes in bugfix releases that prevent idle from running in the 'current' environment.  In private email, Nick agreed with me that with ttk and any possible disablement made optional, it can be added to all current releases. He also suggested being on by default when possible.

I decided not to put anything into 3.5.0.  I intend to start pushing ttk patches perhaps next week after the release branch is split off and the main 3.5 branch is 3.5.1.