It's been many years since I bought a relatively modern iPhone, I'm currently using an SE. I'm looking at eBay listings for iPhone 8s for example, and many of them say that if you buy it for use on an ATT network, it cannot be unlocked. Like it can't ever be unlocked. Why is that? In the past if I bought a locked phone on ATT, I could always get it unlocked, like I did with my SE. What changed?