Why not? Which consumer software breaks?

Normally people say games. I have disabled upnp on my firewall and there're two gaming PCs, a PS3, a PS4 and a PS5 running happily behind it. I just finished a Demon Soul's session with voice chat with friends with no problems. NAT type 2, because I managed my firewall to enable this.

Have you tried a client/server style game not relying on Steam multiplayer?

It makes no difference since as I wrote I manage the firewall to allow this. But yes, since none of the games on consoles use Steam. If NAT wasn't set up I would get NAT type 3 on the PlayStations for example.

ETA: My point was to get an example of something that breaks "because it doesn't work without upnp". I have yet to see a game that doesn't support a fixed set of ports.