NAT is like package delivery IRL. If you’re a server and send a package to a client without NAT, that’s like sending a delivery boy to deliver pizza, goes straight from source to destination.
But with NAT it’s like ordering a package online. It first will be delivered to a distribution center, and then a delivery warehouse in your area, and then the courier delivers packages to all people on his route.
It’s way more complex and you now have a whole bunch of points of failure.
It has already happened. I don’t remember which game but basically they detect the Steam Deck and make an exception only specifically for that.