Even currently supported systems are at risk, probably more so because attackers will assume most people will be running the most recent version of macOS, plus many of them will likely be using the newer systems. Same thing even applies to Windows systems and has been known as security through obscurity since most current attack vectors target things in the current OS that did not exist in older outdated & unsupported versions.
If you follow the advice in the article @John Galt linked (and even wrote), then you will be doing the best to minimize encountering any issues. Make sure to read all the linked articles within that article as well.
As @John Galt has already mentioned, phishing attacks are much more likely to cause you problems than encountering any random issues from using your current system.
If you are still concerned, then clear out the Keychain and each web browsers' stored data (or any other app such as Mail). Or perform a clean install of macOS and only reinstall your game(s) and the game's stored data. Then you have nothing on the system except your game credentials to worry about. However, sometimes people do have issues reinstalling macOS so be careful. I would highly recommend first creating a bootable macOS USB installer just to be safe so you may have more options available if something goes wrong.
Personally I would not worry about it.
