I don't know any extension like this one, but I've used Changes Meter which works really fine. It's a small app in your menubar and gets the job done.
Changes Meter is an app that just notify you about some change in web page. But it does not inform you about what has changed.
I'm using https://urlooker.com, it has free plan, shows exact changes and supports JS-generated pages.
I've found this old software that might do the trick Safari Page Monitor
Using a browser extension for this means it will only notify you when your browser is online.
I use http://changemon.com which notifies me 24x7 and it's quite simple to use.
You don't even have to sign up on the site.