It didn’t get a mention in Apple’s big keynote announcements Monday — which already had plenty of interest to publishers — but deep within Apple’s developer documentation lies perhaps the most important item of all to the news industry.

Adblocking is coming to the iPhone with iOS 9.

Adblocking — running a piece of software in your web browser that prevents ads on most web pages from loading — has moved from a niche behavior for the nerdy few to something mainstream. A report from 2014 found that adblock usage was up 70 percent year-over-year, with over 140 million people blocking ads worldwide, including 41 percent of 18- to 29-year-olds. You can understand why that would be troubling to the publishers who sell those ads. But until now, adblocking has been limited almost entirely to desktop — mobile browsers haven’t allowed it.

Here’s how the developer documentation puts the new change, coming in iOS 9:

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.

Xcode includes a Content Blocker App Extension template that contains code to send your JSON file to Safari. Just edit the JSON file in the template to provide your own triggers and actions. The sample JSON file below contains triggers and actions that block images on webkit.org.

What this means is, when iOS 9 launches in the fall, you’ll be able to go to the App Store and download an extension that will block ads on most news sites.

Is there any chance that won’t be incredibly popular? The desktop version of Safari currently allows a variety of custom extensions, and what’s the most popular? Hint: It’s called AdBlock.

For me, the arguments for using ad blockers range from the unconvincing (dude, information wants to be free) to the reasonable (I don’t need dozens of tracking beacons on every webpage) to the downright understandable (poorly built ads slow my browser to a crawl). I don’t use an ad blocker, but I do block all Flash by default for performance reasons, which accomplishes some of the same ends. The best arguments for adblocking are even stronger on mobile than they are on desktop — bandwidth and performance and battery life are all at a premium.

This is worrisome. Publishers already make tiny dollars on mobile, even as their readers have shifted there in huge numbers. To take one example, The New York Times has more than 50 percent of its digital audience on mobile, but generates only 10 percent of its digital advertising revenue there. Most news outlets aren’t even at that low level.

If iOS users — the majority of mobile web users in the U.S., and disproportionately appealing demographically — can suddenly block all your ads with a simple free download, where is the growth going to come from? (By the way, a version of Adblock Plus for Android just came out a couple weeks ago, though it appears to be more limited than what Apple is allowing.)

The folks behind Adblock Plus — a different ad blocker from AdBlock; it’s a confusing space — aren’t sure they like the way Apple’s allowing it, saying it might make certain adblocking methods harder to implement on the desktop version of Safari. But desktop Safari is small potatoes compared to the web browser on every iPhone and every iPad, where it was impossible to write ad blockers before now.

The potential impact of “Content Blocking Safari Extensions” even goes beyond blocked ads. Apple is explicitly allowing the blocking of cookies on a site-by-site basis. For example, you could build an extension that blocked the cookies that allow a newspaper paywall to work. The Yourtown Times allows you 10 stories free a month? It’s probably using a cookie to keep track of that count. Block that cookie and the paywall comes tumbling down — you’re a fresh visitor every time. Imagine being able to download an extension that blocked paywall cookies on the top 50 paid news sites. It wouldn’t even be particularly hard to code; unless Apple chooses to prevent it, someone will do it. News sites would be able to build workarounds — changing cookie IDs regularly, requiring user login from article 1 — but winning that sort of cat-and-mouse game is something publishers are unlikely to be good at.

Why would Apple do this?

An Apple partisan might argue it just wants to give users control of their iPhone experience, and having debuted extensions in the last version of iOS, allowing them to alter web content is a natural next step.

An Apple realist might argue that its great rival Google makes more than 90 percent of its revenue from online advertising — a growing share of that on mobile, and a large share of that on iPhone. Indeed, Google alone makes about half of all global mobile advertising revenue. So anything that cuts back on mobile advertising revenue is primarily hurting its rival. (Google has been less friendly to adblockers than its “open” positioning would suggest.)

An Apple cynic might note that the company on Monday unveiled its new News app, which promises a beautiful reading experience — and a monetization model based on Apple’s iAds. iAds will, one can assume, never be blockable by third-party extensions available in the App Store. Ads that appear at the operating system level — as opposed to in HTML and JavaScript on a web page — have a rather invulnerable position so long as you keep using Apple products. (It’s good to be the platform.)

Maybe I’m exaggerating the potential impact here. (Talk me down!) Maybe people won’t download the free app at the top of the Most Downloaded list that promises to make their websites load more quickly, more beautifully, and using less data. But use of ad blockers has done nothing but rise, particularly among young users, and people are about to be given an easy way to do on the devices they use most. For the many news companies who are counting on mobile advertising for their future business model, I don’t see a way that this change won’t shave off a real slice of mobile advertising revenue.