Yes it's possible to build ad blockers on Safari using the Apple content blocker API. Support was also added for Web extensions in iOS 15 and macOS Monterey.
The web extension support is what enabled us to deploy complete YouTube ad blocking on iOS in Safari – previously it wasn't possible to do this reliably just with the content blocker API.
The web extension support is what enabled us to deploy complete YouTube ad blocking on iOS in Safari – previously it wasn't possible to do this reliably just with the content blocker API.