tl/dr: Just let flash die.
Adobe officially announced the End-of-Life for Flash a while back. It will officially be EOL at the very end of this year (2020). This means that there will no longer be new or updated flash players after that.
Given the long history of dangerous flash vulnerabilities, using unpatchable software with a history of dangerous bugs is probably a bad idea. The biggest danger (which has happened in the past) are vulnerabilities that allow a malicious flash program to escape its sandbox and execute commands directly on your machine. This would allow an attacker to compromise your machine and install malware simply because you visited the wrong website.
Again, given that such vulnerabilities have happened with Flash before, it's safe to assume that more such issues will be discovered once Flash is EOL and will no longer receive updates. Therefore, enabling Flash as a browser extension is dangerous and, unless you like malware, should probably be avoided. If you really have some hardcore nostalgia for all those Flash games of the 00's, then I would suggest running it in an isolated environment. I.e. consider running in a browser inside a VM that is used for no other purposes and is trashed when you are done. If you continue to use Flash in the long run though you should assume that it will eventually result in malware being installed on your machine without your knowledge.