In Safari 13, Safari Extensions are replaced with Safari App Extensions. The addition of that little word "App" is actually a big deal. Safari App Extensions are faster, lighter and more secure. They run through native Mac apps, meaning they put much less strain on memory and CPU performance. That's great!
However, it also means that Apple made significant changes in the newest version of Safari. Those changes don't allow extensions architected for earlier versions of Safari to run. Which means, we are in the process of developing a new version of the Button which will be compatible with the newer technology.