The iOS WKWebView component available for apps does have a "user scripts" feature in principle lets you do adblocking and other content-munging addons. But yes

The iOS WKWebView component available for apps does have a "user scripts" feature in principle lets you do adblocking and other content-munging addons. But yes it's not a given, might be hard to provide a compatible/full featured API for addons. Let's see what Mozilla does.