Standardized API for themes to add info/links to the customizer
|Reported by:||celloexpressions||Owned by:|
Currently, the theme review team allows themes to inject custom UI with an upsell link into the customizer, as a compromise provided that there is only one link. This creates several problems, such as providing an inconsistent experience for users when switching to different themes and potential breakage if core markup or UI changes. Since the customizer is API-oriented, there isn't generally a restriction to changing the markup or UI in core because custom things added via the API can still be backwards compatible.
If we can come up with a core UI that theme developers can opt into and probably supply a few parameters to, the experience can be standardized for users, developers can keep their links, and we can better encourage proper customization of the customizer via the API.
Technical implementation will be fairly straightforward - a matter of deciding if it should be something along the lines of add_theme_support or a custom section/control type in core that can be added directly. The bigger question is what this UI should look like, what information needs to be able to be provided, and whether it should be on the top level of the customizer or within a section or other nested UI.
See slack discussion: https://wordpress.slack.com/archives/themereview/p1468283558004173.