Four months ago Google announced that they'd be moving to HTML5 By Default to offer a safer, more power-efficient experience. As a reminder, this change disables Adobe Flash Player unless there’s a user indication that they want Flash content on specific sites, and eventually all websites will require the user’s permission to run Flash. To ensure a smooth transition, not all users and sites will be affected immediately. HTML5 by Default and the associated user prompts will be introduced gradually as follows.
The feature will be rolled out to users over a few months. HTML5 By Default will be enabled for 1% of users of Chrome 55 Stable in the next few days. The feature is also enabled for 50% of Chrome 56 beta users. With Chrome 56 stable in February, they plan to enable it for all users.
Starting in January users will be prompted to run Flash on a site-by-site basis for sites that they have never visited before. They want to avoid over-prompting users, so over time they'll tighten this restriction using Site Engagement Index, a heuristic for how much a user interacts with a site based on their browsing activity. In October all sites will require user permission to run Flash.
More details, including specific Site Engagement Index thresholds, are available on the Flash Roadmap Page. Developers can find recommendations on how to test their Flash sites there as well. As sites transition from Flash to HTML5, this change will no longer affect them and the entire web will become faster, more secure and power-efficient.
Source: Chromium Blog