YouTube decided to start detecting ad blockers and refusing to play videos until the blocker is either disabled or users sign up for a Premium subscription.
As 9To5Google reports, Reddit users Sazk100 posted an image of the pop-up that appeared when trying to watch a video on YouTube while also running an ad blocker in their browser. The pop-up states that "ad blockers are not allowed on YouTube," before going on to explain that ads allow YouTube to stay free "for billions of users worldwide."
Users are then given two options: allow ads to play, or sign up for a YouTube Premium subscription which is ad-free, but costs $11.99 per month ($119.99 per year). As PCMag's review concluded, while a YouTube Premium subscription does remove the ads, allows offline downloads, and includes access to YouTube Music, it's expensive.
Not everyone who runs an ad blocker in their browser is going to see this message. A YouTube employee confirmed via Reddit that the pop-up is just an experiment. Whether it gets used more widely will likely come down to how users who are part of the experiment react to it. If most of them either disable their ad blocker or sign-up for a Premium subscription, YouTube will no doubt show the pop-up to more users.
YouTube is at least attempting to make the with-ads experience more palatable by killing off those annoying overlay banner ads. It's also trying to make the Premium subscription more attractive with experimental features such as higher-quality 1080p video playback.