Fast-forward to the present and Apple is, in many’s opinion, dooming x‑callback‑url with its newest approach to URLs. Doom-mongers should have arrived sooner, though, since most of measures Apple took that could knock x‑callback‑url off were implemented in iOS 8 with Extensions. I predicted that a native alternative would appeal to developers and cause the decline of x‑callback‑url implementations in new apps. - One Tap Less
Interesting discussion. The URL scheme hack was never officially allowed by Apple, but by ignoring it for this long, they've allowed the system to grow to a blossoming indie platform. It's certainly not known by the greater public (although most of them will probably have experienced a Dropbox login or Facebook redirect that works thanks to URL schemes), but for those of use who know it, it made iOS a lot more efficient and productive.
But since iOS 8 I think the writing is on the wall for the end of this hack. Although Apple gives multiple reasons for locking down the schemes, I think a lack of platform control is what motivates them the most. One one side you have Twitter and others that exploited the system to collect data, on the other side you have the burden of them not supporting an unsupported hack that, if it accidentally breaks without an officially sanctioned alternative, will upset a lot of engaged users and break a lot of apps.
With extensions in iOS 8 and the new user activity API's in iOS 9 they've created a build-in alternative to url-schemes that can replace a big part of the functionally in a supported way that also guarantees user privacy.
I think iOS 8 was a wake up call, iOS 9 the final warning, and that iOS 10 will completely kill unsanctioned or undocumented use of url schemes. We will lose a lot of functionality, but by making inter-app-communication a feature Apple brings the hack to the masses. Yes we'll lose features, but we will win the certainty that a big part of this concept will live on after Apple kills the original exploit.