Apple has gone on an un-signing spree today. The company has to sign a version of iOS for user to be able to install it and usually continues to un-sign the firmware for a few days after releasing a new version of iOS allowing users to downgrade using a computer if there are any major problems.
Jailbreak users often use this to downgrade if they are awaiting a new Jailbreak release. Today Apple has stopped signing several iOS versions:
iOS 12.4.9 (16H5)
#tvOS 8.4.3 (12H903)
#iOS 14.2 (18B92)
bridgeOS 4.6 (17P6610)