I don't think it is as late as December 2020. Firefox, for example, have said they will drop support before the end of 2019.
Firefox is not dropping support for Flash in 2019. What they will do, in Firefox 69, is drop the 'always activate' option.
Where do you get the December 2020 figure?
See https://developer.mozilla.org/en-US/docs/Plugins/Roadmap for more information.
- Similar road map for Chrome is available at Flash Roadmap - The Chromium Projects
- Soon-to-be old Microsoft Edge: https://blogs.windows.com/msedgedev/2019/08/30/update-removing-flash-microsoft-edge-intern et-explorer/
- New, Chromium-based Edge will follow Google's Chromium road map.
- I've not found similar road maps for Safari (Apple is known for their secrecy), but it's safe to assume they'll follow similar pattern.
Essentially, all major browser vendors have indicated they will support Flash until the end of 2020, however, they will continue to make it increasingly difficult to use Flash (e.g. blocking/disabling by default, removing 'always allow' options, deleting white-lists upon terminating a browser session, etc).
With that said, it is incumbent Flash/Flex developers convert their Flash content to some other technology prior to the end of 2020.