You should ask in the InDesign SDK forum:
My impression is that Adobe is phasing out the use of Flash within applications. For example, Photoshop and InDesign have discontinued Flash-based panels like the MiniBridge panel. But I'm not a developer so I couldn't say for sure.
Thanks for your quick reply, i have forwarded the question to the InDesign SDK forum
you are right that plugin development is discussed in the SDK forum.
There is a big difference though between plugins - built using the C++ plugin SDK (hence the traditional name of the forum) and extensions that utilize various other technologies. Those all end up at the scripting interface and are thus better discussed at the scripting forum (for InDesign specifics) or at forums dedicated to their special technology.
Keywords for those other technologies include AppleScript, VisualBasic, ExtendScript, PatchPanel, Flash or Flex Extension, CSXS, ExtensionBuilder, Creative Suite SDK, CEP, ZXP, CC Extensions, HTML Extensions. Probably I missed some more, as my focus is on C++ plugins.
You are also right that Flash Extensions are phased out, see the link to HTML Extensions. Therefor the choice between those technologies depends the targeted application version, for the latest CC as of 2016 it apparently is HTML Extensions.
Thanks, Dirk. It's a bit at the fringes of my knowledgebase, but it's good to know I had the basics right.
Sorry, Steve, you got me wrong.
Please don't stop reading after the trigger / keyword "Plugin".
When you see that, please first watch out for the other keywords, e.g. Flash.
Plugin and C++ => SDK
All others => Scripting, or the other links