Earlier quoted context omitted.
Icab mobile does this. It's good. The problem is, safari is better for a lot of things, and also has native adblocker integration. Would be better to be able to designate a desktop user agent in safari as well.
WKWebViews support the same JSON content blocking lists that Safari uses for its content blocking extensions, so while you can’t support the same extensions as Safari you could at least bundle block lists with the app.
Firefox focus is the only third party browser I know of that has a native content blocker.