Will cardhunter be ported or dead in 2020? https://blogs.adobe.com/conversations/2017/07/adobe-flash-update.html
I assume that the Steam version will continue to work, because it includes a stand-alone client which presumably doesn't really need to be updated. ... But it would be best if there was a real port.
Steam build will keep trucking, and unless Adobe have a secret kill switch I can't see why older browsers wouldn't keep working. I have opinions about HTML5 but this probably isn't the best place for them.
The main issue I see with Flash ending is security updates. Browsers have already been turning off Flash by default, and if browser makers believe it to be too high a security risk for users, you know they won't want compatibility. This won't stop unofficial add-ons and things, but ~shrug.~ I won't get into any huge HTML5 debate, don't worry, but I'll just say (having developed a game or two in it) that I know there have been browser compatibility issues, but the gaps keep narrowing (mainly with WebGL, and with MSIE being deprecated/obsoleted), and the big thing on mind, I figure by 2020, it'll be even stabler. Just talkin' abstracts here.
Firefox just made me update, and failed to mention they were going to turn off all my plug-ins. I just spent 15 minutes trying to get Adblock working again, and even that will probably stop for good in November. And it wouldn't surprise me if Google and all its websites were to suddenly stop allowing people to use older browsers. Remember when it was possible to do a web search without first setting up a secure connection? I'm already sitting here with two browsers open all the time because some sites only work in Chrome, but I hate it's interface.