As to Adobe Flash Player, the stand-alone version (has a GUI, no browser needed) is available as a Flatpak.
Below is the information, and a screenshot of it running the old flash game Incredipede:
ID: com.adobe.Flash-Player-Projector
Branch: stable
Version: 32.0.0.465
Installation: system
Installed: 7.1 MB
offline games but what about online games?
Maybe. There is an address bar to enter a URL. I imagine the target would have to be an .swf file, though. You would have to do a test and find out if you can play without downloading the file.
It works for some games but not all games what are on servers another companies online.
I hate that Adobe stopped .swf, many games risk to be lost for this, this is why I prefer downloading my favorite ones. And in the meanwhile helping to add games on FlashArch. Topaz suggested a good way to play them without using the original Flash player in .exe format (that I use with Wine), but personally on that player resizing the window makes some writing fields on games unusable and some keys stop being detected and audio is played after a short delay (Issues · flathub/com.adobe.Flash-Player-Projector · GitHub). It's funny, the original .exe player version works fine but with limited FPS due to Wine emulation and the Linux version has good/perfect FPS but problems that make playing unenjoyable 
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The problem the people who palyed some Adobe Flash players are not supported on linux and that why people don't changed operating system because a games.
