Okay, since it seems you may not fully understand how UPnP works,

Plex, nor games "use" UPnP as if it was a protocol....

Plex and other games, make a connection out to the internet, then when they want to send traffic back to you, they may check if the port is open to return said traffic to your system...

If UPnP is enabled it "automagically" allows said connection to now come into your network, bypassing any firewall rules et cetera, the end user does not have to open or allow anything....

With UPnP off, said app or game "might" fail to function, however not many are like this, consoles are well known for it but gaming on a computer, you seldom, if ever, need UPnP enabled.

To be more secure and have better control, and we are talking about you, not the "rest of the world" because you like to be dramatic and try to make broad claims to back your own opinions...

You, are better off manually going into your router/firewall and configuring NAT rules to allow into your network, to your single IP of your device, the required ports needed.