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. The app has been changed to run on Safari 13+.