That's pretty much what RandomGuy said, it draws a lot of processing power for what it does and it apparently is a nightmare to debug and is a vulnerable piece of software, allowing malevolent hackers to put malware directly through your browser sometimes.

Basically, it's just a very old and outdated software, it's not even really used anymore by anything. Most "flash" games today aren't made in flash anymore, but directly in HTML5 or other stuff. But a lot of old games and animations did use flash and won't be able to work normally in 2021.

You can find a few organizations that tried to save the major ones, check out Newgrounds or the Flashpoint project, you can still find a ton of stuff there.

From what I've been told, Flash is extremely outdated while there are multiple software that does what Flash does better. Making it not worth keeping it running.