OneNote VSTO gone?

VSTO does not support creating addins for OneNote. You can crate an addin for OneNote in a slightly different way though. You need to implement the IRibbonExtensibility interface as noted here.

This is very helpful tutorial for anyone looking for a guide on how to create an add-in for OneNote. Thanks to the author of Onetastic for Microsoft OneNote (Onetastic Add-in) in sharing this guide. Here is the link for the guide: Creating OneNote Add-ins. This guide is very informative.