Apple has stopped signing iOS 12.4 which means that it is no longer possible to downgrade or restore to that firmware version.
Currently, Apple's latest firmware is iOS 12.4.1. Apple released that update a couple weeks ago to kill the Sock Puppet exploit originally discovered by Ned Williamson.
If you accidentally update or restore, you'll find yourself on iOS 12.4.1. Developers have been seeded with iOS 13 GM Seed and iOS 13.1 beta 3.
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