With Flash at end-of-life, I'm testing an alternative for handling Flash on the forums, called Ruffle.
It's under active development and still only has preliminary support for some features, and is missing some important ones (ActionScript 3 being by far the biggest, though it'll hopefully be added soon), but it seems promising and fairly easy to integrate with the forums. You need to have a reasonably up to date web browser with support for web assembly (big name browsers like Chrome, Firefox and Safari should all work, though there appear to be current issues with Chrome on macOS).
If you'd like to give it a try, go to your User Control Panel and under Board Preferences change your style to "ruffle_test"; this is identical to the default prosilver style but has Ruffle enabled for all SWF files on the site.
Alternatively, you can try out Ruffle with the files I tested against:
This is for testing only, it likely won't work right with many Flash files, and won't work properly at all with any that use ActionScript 3.
I'll try to keep this topic updated as I install new versions of Ruffle, which I'll do when they announce major fixes and improvements.