You can no longer downgrade iOS 15.0.1 and iPadOS 15.0.1 back to iOS 15.0 and iPadOS 15.0 on your iPhone and iPad.
Since Apple is Not Signing iOS 15.0 or iPadOS 15.0, You Can No Longer Downgrade from the New 15.0.1 Update
The reason for this is simple – Apple has stopped signing the initial firmware release of iOS 15 and iPadOS 15. If you’re already on the firmware, then you have to update to iOS 15.0.1 and iPadOS 15.0.1, eventually. You cannot restore your device to iOS 15.0 or iPadOS 15.0 using Finder or iTunes. It has to be iOS 15.0.1 or iPadOS 15.0.1 at this point.
iOS 15.0.1 is an extremely important update for iPhone 13 owners. It fixes a bug which prevents the Apple Watch from unlocking the iPhone while wearing a mask. If you’ve been having problems on that end therefore it is highly recommended that you update to the latest firmware release instantly.
Last but not the least, make sure you download the correct firmware file before going ahead with a clean install of the firmware. If you use the iOS 15.0 or iPadOS 15.0 firmware files for restoring your device, it will simply fail.
Source: wccftech