this might help Esri’s Roadmap for Web Developers | ArcGIS Blog
Our organization decided not to support FLEX even before ESRI decided to move away from it. The main reasons were security issues with the Flash Plug-in and the lack of support on mobile devices.
I will also add that when I was developing FLEX, there was a steady stream of updated Flash Players getting pushed in our organization, largely because of security holes in Flash. Every time they pushed it, it broke my debug environment. There was a debug version of the Flash Player I needed instead, which was replaced by the new standard Player every time they pushed an update. I was constantly having to find the appropriate debug equivalent for the newest Player. It was a pain!
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