Actually, I figured it out, when an app I wrote, that uses the keychain, started allowing me to log into the app, using Sign in with Apple (which has some stuff that is only available when the login is set up), on devices that were not the ones that I set up.

In my case, I liked that, and so will my users.

But I do think that it could be problematic, if this means that authorities could now get ahold of your keychain, when having it restricted to a single device, avoids that.