https://wccftech.com/apple-is-no-longer-signing-ios-12-2-downgrade-from-ios-12-3-not-possible/

Apple has stopped signing iOS 12.2, meaning that you can no longer downgrade iOS 12.3 regardless of how hard you try.

It’s only a matter of time Apple stops signing a particular firmware version, thwarting the downgrade process altogether. And right now, the company has gone ahead and closed the iOS 12.2 signing window completely, meaning that you can no longer downgrade iOS 12.3. Although iOS 12.3 is a great little update that brings with it the all-new TV app and whatnot, but for those who were hopeful that they would stick with iOS 12.2 in hopes for a jailbreak, then your hopes and dreams just turned sour very fast.