The short version of what's affecting the Flippity templates is a change to some of Google's security policies and how it handles data published via Google Sheets. You can read the full explanation here on the Flippity site.
Fortunately, some of the Flippity templates are working again. And there are some templates in the "Skip the Spreadsheet" section of Flippity that work without using Google Sheets at all. In the meantime, I'll just be patient as I wait for my favorite Flippity templates to be restored to full functionality.