I like to think I discoverd a "hidden" feature or easter egg (like when you tap some phone settings value X times and then the phone becomes "specially activated", or key in some numeric keypad mantra that unlocks a carrier settings, or what not), but maybe this NAT64 thing is somewhat widely known, haha.
So story goes, I was setting up WiFi sharing, and the menu locked up, so I went to Force Quit it (Righ click dock icon, then hold down Option key). As I did that I noticed a partially occluded new option appear beneath the normal "Share to" options panel, that seemed like you could scroll down to it. Then it force quit and disappeared.
From the quick glance, I thought it said "Create NAT4 network" and this got me excited because it sounded powerful and new. I knew of NAT, but not much about it, other than: "Maybe I can create a LAN where devices on this wifi have their own IP but we all share the public facing one?" Or some approximate impression like that.
Now I wanted it back, but I didn't know how to get it back, because I wasn't exactly sure how I had triggered it, or it it were "special state" that had surfaced only in the context of the lock up error of the settings page.
I tried holding Option while on the sharing page. No. Tried it while right click the information circle next to WiFi. Again nothing.
I searched AI but the ways it found to activate this didn't work. There was some talk of CLI but I wanted this GUI thing. Apple had clearly intended there to be an menu option, and I wanted it.
Finally, I tried to Force Quit again, to see if that did anything. But before I clicked Force Quit, something had happened when I hit Option: then menu had returned. I cautiously released the Option key and the menu persisted. Yay! I could now toggle it on. Which I immediately did. I have no idea what it does, really, but it's cool. A hidden option on Internet Sharing is now enabled. Excellent.
So hopefully you had some fun reading this or trying it for yourself. And maybe you can tell me what exactly this does. Finally, I wasn't aware that "Option when Right-clicked Dock Icon" as a "UI idiom" on macOS for exposing new behavior in the apps themselves, I thought it only worked to bring up new items in the Right click menu. So that's interesting. And why I think maybe I discovered something new or undocumented. So hopefully I did and you have some fun with it. This was all on the latest version of macOS 26.
[0]: https://support.apple.com/en-mn/guide/mac-help/mchlp1540/mac (there's this guide, to be sure, but the suggested method to enable to hidden menu does not work)