Since Adobe Flash is going the way of the Dodo Bird, I thought I'd check up on Open Source Alternatives, since I have some Standalone .swf files (games, etc).
Two came up in my searches, Lightspark (which seems to be, mainly a Plug-in), and Ghu Gnash, which I like better, since it works as a Standalone Program. Unfortunately, it doesn't seem available in Bullseye (and Search didn't find it for Buster either). It DOES show up in Stretch, but that's on a different machine than where my .swf files are.