Not quite. Safari extensions were, like other browsers, a package of JS that could mostly run as it liked in your tab. The new app extensions system is more gen

Not quite. Safari extensions were, like other browsers, a package of JS that could mostly run as it liked in your tab. The new app extensions system is more generally applicable across the system, but more precise in what it allows. For example, it allows an app to provide a sharing target – so that other apps can share content to it. The relevant thing that's provided here is a content blocking extension point. Unlike ad blockers providing a bundle of JS that may or may not block ads, record your browser history, etc, the content blocking extension point is a pull-based system. Safari asks the app for its list of content to block, then computes that into a fast lookup table of some sort, and never talks to the app during browsing. This means ad blocker app built with the app extensions API is more secure, more private, and in all likelihood, faster. Potential complexity is reduced, but that may be worth it.