Flash player alternatives after 2020 on Mac?

Here's the official EOL announcement and FAQ: 

https://theblog.adobe.com/adobe-flash-update/

https://www.adobe.com/products/flashplayer/end-of-life.html

Harman has a number of solutions for enterprises that need to use Flash for legacy reasons; however, it's worth noting that those solutions are not free, and are not intended for the average individual (either from a price perspective or a technical solution perspective).  The enterprise cases are more about either a software developer needing a post-Flash solution that they can distribute to customers, or a large enterprise with a critical application that must continue to run securely over the long-term.  It's not really intended to solve the general-browsing problem.  The point is to move the web to standards-based, browser-native technologies.

Adobe AIR (which has been retired for several years and is also maintained by Harman) was the recommended solution for building desktop and mobile app experiences on top of Flash technology.  Harman still licenses and maintains the AIR SDK for developers that are reliant on it, but if you're going to invest in a porting effort, it's probably worth just making the jump to a forward-looking technology stack for maximum longevity.

the alternative is HTML

1. There has never been a Flash Player for iOS. iOS runs on iPhone and iPad. If you have a Mac you are running macOS. 
2. Flash Player is going away for all systems as you note, at the end of the year. 
3. There has been 3 years to re-make or recreate necessary content. 
4. Harman will offer software assistance, including continuing support for a limited time for Flash Player, for Mac and Windows. Doesn't seem likely they will offer anything for Android but ask them. Certainly they will not offer anything for iOS. Price is by negotiation. Since people are taking about a 5 figure cost, getting your system right is pretty important!