https://wccftech.com/apple-has-stopped-signing-ios-12-1-4-but-you-dont-have-to-worry-about-anything-at-all/

Apple has stopped signing iOS 12.1.4, meaning that you can’t downgrade or upgrade to said firmware using iTunes. But that is not a problem at all.

iOS 12.2 is the current public release for iPhone and iPad devices out in the wild. Given that Apple had released a new firmware, it was only a matter of time Apple would stop signing iOS 12.1.4, killing the downgrade or upgrade process altogether. And right now, if you were thinking about downgrading back to iOS 12.1.4, or upgrading straight to it from an older firmware, you can’t do it, as Apple has stopped signing the aforementioned firmware.