Two years ago, Apple launched an aggressive battle against ads that track users across the web. Today executives in the online publishing and advertising industries say that effort has been stunningly effective—posing a problem for advertisers looking to reach affluent consumers.

Since Apple introduced what it calls its Intelligent Tracking Prevention feature in September 2017, and with subsequent updates last year, advertisers have largely lost the ability to target people on Safari based on their browsing habits with cookies, the most commonly used technology for tracking. One result: The cost of reaching Safari users has fallen over 60% in the past two years, according to data from ad tech firm Rubicon Project. Meanwhile ad prices on Google’s Chrome browser have risen slightly.