Backwards compatibility is never certain. Nothing stays the same forever because technology is constantly moving forward.
Everyone had 3+ years advance notice that Flash Player was ending, more than ample time for programmers, developers and IT experts to replace their flash-based content.
July 2017 Announcement
https://theblog.adobe.com/adobe-flash-update/
Flash Player EOL
https://www.adobe.com/products/flashplayer/end-of-life.html
Flash Player EOL info for Enterprise
https://www.adobe.com/products/flashplayer/enterprise-end-of-life.html
I think it's fair to say that unless Rosetta intends to retrofit their old software to run without Flash Player, it's highly unlikely that you will find solutions here. Flash Player ended because Facebook, Google, Microsoft, Apple, Mozilla and other browser vendors killed it for security reasons.
https://www.maketecheasier.com/browsers-ending-flash-support/
NOTE: Running old versions of Flash Player that no longer receive security patches is risky for you and your computer. Don't do it.
[Moderator moved from Using the Community (forums) to Flash Player.]