Your operating system, browser and Flash Player are all primarily tasked with processing untrusted content. It's important for those products to stay abreast of the latest research and intelligence from the industry. On December 31, that work will stop, and bad guys will eventually deploy automated attacks to look for and exploit unpatched versions.
You should not use an old Flash Player (or an old browser) to browse the web moving forward. If you want to keep an old configuration for archival purposes, you can do that; however, there's no reason for content providers to pay to host content on the web that nobody can play, and you really should isolate your archival stuff from the Internet anyway. There are many ways to go about this, but none of them are trivial, and it's beyond the scope of what we're going to support here.
After Jan 12, 2021, Flash Player will only load content that you explicitly allow via AllowListUrlParameter directives. Most major browsers will be dropping their plug-in interfaces by at that point as well, so there will be no way to run a plug-in in a current browser.
You can find more in the EOL FAQ, here:
https://www.adobe.com/products/flashplayer/end-of-life.html