Open standards such as HTML5, WebGL and WebAssembly have continually matured over the years and serve as viable alternatives for Flash content. Also, major browser vendors are integrating these open standards into their browsers and deprecating most other plug-ins (like Flash Player). See Flash Player EOL announcements from Apple, Facebook, Google, Microsoft and Mozilla.
By providing more than three years’ advance notice, Adobe believes that there has been sufficient time for developers, designers, businesses and other parties to migrate Flash content to new standards. The EOL timing was in co-ordination with some of the major browser vendors.