Google has been very transparent about about their roadmap for deprecating Flash Player, and all of the major browser vendors and Adobe are working together towards a graceful deprecation of Flash Player at the of 2020. I think it's safe to say that Google's engineering blog is an authoritative source for information.
In the interim, Google is encouraging content providers to transition to HTML and JavaScript by increasing the amount of friction that end-users experience when trying to use Flash-based content in their browser. Other browsers are taking similar approaches, and have published their own roadmaps. In a similar vein, Adobe has announced that Flash Player will reach its end-of-life at the end of 2020.
Adobe provides a number of tools for either converting or producing replacement standards-based content, which developers can take advantage today. Wherever possible, we'd strongly recommend that content providers begin that migration effort, if they haven't already.