Normally old versions of ios are signed a week after releasing a new version, it makes no sense to stop signing a version even before releasing a new one.
Unless they found a zero day. That would cause this immediately.
As a matter of fact, this whole episode would be consistent with Apple finding a serious bug / zero-day in 15.0.1 and the 946 build.
No way to know for sure and don’t want to start rumors. But if they did find one, I think this is what it might look like.
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