Well, here it is, a month before the "deadline", and i've just completed a fairly extensive review of the current most popular, featured, and new themes, and guess what? A tiny fraction are Customizer based, and many many continue to use "Theme Options" on the Appearance tab. So, there is clearly strong resistance to this new requirement - or there will be a giant backlog of revised themes in the next month. Look out!

Perhaps most of these themes plan to us the plugin alternative - put all the options in a plugin. The most interesting reaction I've seen to this requirement is with Eclipse 2, which has an almost identical set of options in Customizer and off the Appearance menu.

But there are still a whole lot of themes Appearance Theme Options based. Sort of a worry since the deadline should be early October.