Google officially announced the date when the advertisers lose the ability to use Flash in the Google Display Network, and advertising at 100% will move to HTML5.

From 30 June 2016 advertisers will not be able to upload a new ad in Flash format, but the old is still displayed.
2 January 2017 even the old Flash ads will be completely disabled and advertisers need to the time to convert all your banners to HTML5.