I think this update wasn’t mentioned during this years’ Apple WWDC keynote, but it’s publicly available in the Safari 9.0 prerelease notes:
The new Safari release brings Content Blocking Safari Extensions to iOS. Content Blocking gives your extensions a fast and efficient way to block cookies, images, resources, pop-ups, and other content.
Your app extension is responsible for supplying a JSON file to Safari. The JSON consists of an array of rules (triggers and actions) for blocking specified content. Safari converts the JSON to bytecode, which it applies efficiently to all resource loads without leaking information about the user’s browsing back to the app extension.
I take it this means developers will be able to make ad-blocking extensions for mobile Safari! Looks like Apple is prepared to back up their firm stance on user privacy with some very concrete actions.
If publishers and ad companies are worried about the rising use of ad-blockers now, this is only going to get worse when people have the option to install one on iOS.