I'm not a developer so I can't speak to the details of this. But I believe with CC 2014, the Extensions panel was replaced with Add-ons. In James Wamser's InDesign New Features document, I see these entries:
* * *
InDesign CC (June 17, 2013):
Adobe Exchange panel
The new Adobe Exchange panel is now an in-app experience for you to explore and discover content, plug-ins, and scripts that you can purchase or download for free. Replaced by Adobe Add-ons: (see Page 78) in InDesign CC (2014).
InDesign CC 2014 (June 18, 2014):
Under the Window menu, you can now choose Browse Add-ons. This feature takes you to a new Adobe Add-ons
page of the creative.adobe.com website to let you find, install, and remove InDesign plug-ins and extensions of
* * *
InDesignSecrets.com has an Add-ons forum which might have more information:
I remember seeing mention of this during prerelease, but most of it went over my head.
n InDesign CC (2014).
From what I am reading, you take the same extensible InDesign commands and wrap them up in a new garment made out of HTML code instead of the older Flash code. But I am not reading much. The few tools from 2014 appear to be not current with CC 2017.
This looks nearly exactly what I need to read. Many thanks again!
Mike Witherell in Maryland