It’s absolutely required for most multiplayer games. Many need random ports and some even refuse to work if UPnP is blocked even if you manually open a port for them.
I've never had UPnP enabled and I don't have any problems doing online gaming / flight sim / video chatting / etc.
Same. I've found the biggest problem was SNAT rewriting the (source) port number. netfilter, by default, doesn't do this. pf does but you can configure it not to.
On series X you can set up port forwarding really easily. I had to do it for openwrt