Using Adobe Flash offline after 2020?

Flash Player is going away.  The browsers are simultaneously removing support for plug-ins, so even if you had Flash, it won't run in a modern browser post-EOL (Safari 14 has already dropped plug-ins).  There is a stand-alone version of Flash Player for development and kiosk applications that some folks use for games.  It really depends on how the content is authored and packaged as to whether or not that's workable.  

No version of Flash Player will be available for download after EOL, and at some point, future changes to the operating system will probably result in old versions of Flash Player being unusable.  A lot of the work we do on a daily basis is the invisible work of keeping up with the changes in browsers and operating systems that would otherwise break Flash.

Long-term archival issues, like keeping old software alive are a whole area of expertise (interviews with folks at places like the Tech Museum and art curators trying to keep tech-based art installations alive are really interesting).  Virtualization is great for a lot of this.  Solutions like VMWare and VirtualBox allow you to create images of old operating systems and software, and do the low-level work of keeping them running on modern hardware and operating systems. 

You don't really want to run an outdated browser or Flash Player on your day-to-day computer.  You'd be better off making a virtual machine image that you keep disconnected from the internet.  You could keep all your old stuff on it, and it would work, without exposing you to all of the security headaches related to running unpatched stuff while browsing the web.

You should definitely get that done before we hit EOL, as the downloads will be going away, and getting software from untrusted third-party sources is inherently sketchy.