LAX has it, I've been through on the London to Auckland route, both ways.

Both times they didn't have enough staff to man the separate customs desk, and ran us through a general customs hall.

It's been and gone a few times over the years on that route. At times immigration but no baggage claim, at times transit lounge (actually I'm not sure I ever did go through when that was in force, I think it came and went in-between trips for me), at times full re-check-in... anyway, I think the setup was different every time. And every time they found a way to make it a PITA. I eventually got fed up with it, and don't travel that way any more.

I prefer via Dubai, Hong Kong, or Shanghai. Border control officers there, while still clearly doing their job meticulously, manage to be polite as well.

There are a few other airports that do, too, but it's uncommonly used.

Interesting, as far as I seem to remember all the airports in Europe I've been to had this concept, except tiny regional airports. Probably because traditionally pretty much any connecting flight involved transit through a third country that you'd need a separate visa to visit.

The idea is that you "enter the country" when going out from the main part of the airport, but you can go from any arriving flight to any departing flight without doing so, and most of the facilities - shops, waiting rooms, etc - are 'inside the zone' as well.