The theme review team is putting Customizer code examples on Github, according to an announcement today:
The first area to tackle is example Customizer code, which currently includes examples for adding panels, sections, settings, basic core controls, specialized core text controls (email, URL, etc.), advanced core controls (color control, image control), and an example custom control (radio image).
You can check out the GitHub here, where the team has an entire account dedicated to theme materials.
I’m happy to see this, though I hope some of these practices end up back into the theme handbook as well.