For completeness, here's the original announcement from 2017:
https://theblog.adobe.com/adobe-flash-update/
Here's the EOL FAQ:
https://theblog.adobe.com/adobe-flash-update/
Like all things, software has a finite lifecycle. For developers that want to (or have financial incentive to) keep their content alive on modern technology, they've had ample notice to migrate that content to modern technologies.
To audit what's going to break, you could set your system clock ahead to something like Feb 1 2021, and see what happens to the things that you really want to keep running.
There are long-standing approaches to running archival software. This isn't a new problem, but it's beyond the scope of the support that we're going to offer here.