According to a spokesGoogle the Google Display Network and DoubleClick Digital Marketing are now going 100 percent HTML5.
From June 30, 2016, Google will no longer accept new Flash display ads from advertisers. On January 2, 2017, even old Flash ads will be blocked from appearing, making Google’s ad network mostly Flash-free.
Video ads will remain in Flash however, although there are not many of those.
Google has been trying to get advertisers off of Flash for some time, providing tools and best practices for switching. Now it apparently feels good enough about the switch to force it on the world.
Even Adobe has had enough of Flash. It killed Flash Professional—its Flash authoring tool—in December, and today announced Adobe Animate, which outputs projects in HTML5.