FoldingText current official release 2.2 (770).
Selecting “Software Development Kit” from the Help menu navigates to here:
file:///Applications/FoldingText.app/Contents/Resources/dist/sdk/docs/tutorial-Plugins.html
Near the bottom of this page there is an entry
- Learn from the existing plugins and use them as starting points for your own solutions.
which links to this URL
https://github.com/FoldingText/plugins/tree/FoldingText-Dev
That in turn generates a github 404 Error.