https://wccftech.com/apple-no-longer-signing-ios-14-4-2-and-ipados-14-4-2-stops-downgrades-from-14-5-and-14-5-1/

You can no longer downgrade to iOS 14.4.2 or iPadOS 14.4.2 as Apple is no longer signing the said firmware. Here is what you can do.

Apple has stopped signing the iOS 14.4.2 and iPadOS 14.4.2 firmware for both iPhone and iPad. What this means is, you can no longer downgrade to said firmware anymore if you have updated to the new iOS 14.5 / iPadOS 14.5 or iOS 14.5.1 / iPadOS 14.5.1. The interesting thing here is that you can jump between 14.5 and 14.5.1 as both are being signed by Apple currently.