This isn’t about signing the OS updates. This is about the device contacting Apple’s activation servers during initial setup.
In the linked thread, the users have a “working” device with software already installed, but the activation fails with a cryptic “Activation Error” without any reference to the fact that they must update.
Typically, Apple continues to allow activation on older OSes, as long as they were installed previously. I’ve intentionally kept some devices on older versions, and personally have never had a problem activating after a reset. But I suppose that’s only possible because Apple allowed it.
EDIT: I was wrong about the "Activation Error" not mentioning a software upgrade. It does mention it. But my point about Apple controlling access to the software still stands. It's something weird that Apple did specifically for iOS 9 and the iPhone 6s. Naturally, who really cares about one random version combination? I don't in this particular case, but the fact that Apple can control it is weird to me.
In the linked thread, the users have a “working” device with software already installed, but the activation fails with a cryptic “Activation Error” without any reference to the fact that they must update.
Typically, Apple continues to allow activation on older OSes, as long as they were installed previously. I’ve intentionally kept some devices on older versions, and personally have never had a problem activating after a reset. But I suppose that’s only possible because Apple allowed it.
EDIT: I was wrong about the "Activation Error" not mentioning a software upgrade. It does mention it. But my point about Apple controlling access to the software still stands. It's something weird that Apple did specifically for iOS 9 and the iPhone 6s. Naturally, who really cares about one random version combination? I don't in this particular case, but the fact that Apple can control it is weird to me.
Even more concerning is that some users report that the device locked them out _even during normal use of the phone_: https://www.reddit.com/r/iphone/comments/acmytg/iphone_6s_pl...