I'm curious why Adobe stopped developing this technology and decided to shut it down in 2020?
Short Answer: Flash Player is NOT MOBILE (although people found ways to make it mobile). HTML5,
WebGL, and Adobe AIR are mobile.
Flash player is an unnecessary burden on the user. And it didn't evolve as it was supposed to.
That is what I think was the final nail in the coffin.
My answer is that Facebook, Google and Apple wanted full control of the web and financially encouraged Adobe to make it happen.