How likely is it that such software-activated fuse-based kill switches are built into iPhones? Any insights?

So this article isn't about a kill switch, just blocking downgrades and custom ROMs.

But to answer your question: we know iPhones have a foolproof kill switch, it's a feature. Just mark your device as lost in Find My and it'll be locked until someone can provide your login details. Assuming it requires logging in to your Apple account (which it does, AFAIK; I don't think logging in to a local account is enough), this is the same as a remote kill switch; Apple could simply make a device enter this locked-down state and then tweak their server systems to deny logins.

100%, if you steal a phone from the Apple store they just remote brick it.

Apple has been doing that since forever and will remotely kill switch devices so they need to be destroyed instead of reused: https://fighttorepair.substack.com/p/activation-locks-send-w...

Millions of fully working apple devices are destroyed because of that even - Apple won't unlock them even with proof of ownership.

I'd say for commercial hardware it is a near certainty even if you won't ever know until it is much too late.

Realize that many of these manufacturers sell their hardware in and employ companies in highly policed societies. Just the fact that they are allowed to continue to operate implies that they are playing ball and may well have to perform a couple of favors. And that's assuming they are fully aware of what they are shipping, which may not be always the case.

I don't think it is a bad model at all to consider any cell phone to be compromised in multiple ways even though you don't have hard proof.

It's there on all phones since forever lol. Apple can ship an update that adds "update without asking for confirmation" tomorrow and then ship another one that shows nothing but a middle finger on boot and you would not be able to do anything, including downgrading back.

The M-series CPUs found in iPads (which cannot boot custom payloads) are the same as the M-series CPUs found in Macbooks (which can boot custom payloads) - just with different fuses pre-burnt during manufacturing.

Pre-prod (etc.) devices will also have different fuses burnt.

iPhones already cannot be downgraded, they can only install OS versions signed by apple during the install time. (search SHSH blobs) They also can't run unsigned IPA files (apps). Not sure if they have a physical fuse, but it's not much different.