"We recently announced that the Google Feed API would be retired. As a result, the Slideshow gadget will no longer function and we will remove it from your blog by 29 September 2016."
Funny how Google announces changes to its blog "features". See https://developers.googleblog.com/2016/06/announcing-turndown-of-google-feed-api.html . It seems that they withdraw features more often than they introduce new ones. "Turndown" means the feature is going! Sorry, "retired" is the euphemism I should be using.