After the official version of iOS 15.1 was released in late October, Apple today stopped verifying iOS 15.0.2. This means that users who update to iOS 15.1 can no longer downgrade to iOS 15.0.2.
iOS 15.1 added the simultaneous sharing function, and also added the function of shooting ProRes videos on the iPhone 13 Pro and iPhone 13 Pro Max. At the same time, a verifiable COVID-19 vaccination card was introduced in Apple Wallet.
Apple usually stops verifying the old version of iOS within a few weeks after pushing the official version update.
For some users who have updated to the latest iOS system, when they encounter major bugs, the downgrade method is sometimes very effective.
This is why Apple will wait for a while before closing the verification channel of the old iOS system.
In addition, Apple iOS 15.0.2 fixes the AirTag may not be displayed in the “Items” tab of “Find”, the use of “Finder” or iTunes to restore or update iPhone 13 models may fail issues, etc.
Hazel Yu 
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John Shen 
Sean Lee