Following the release of iOS 12.4 earlier this month, Apple has now stopped signing iOS 12.3.1 – iOS 12.3.2. This means one cannot downgrade back to iOS 12.3.1 – iOS 12.3.2 from iOS 12.4 now on their iPhone or iPad.
The only way for one to downgrade from iOS 12.4 to iOS 12.3.2 is if they saved the necessary SHSH blobs.
Apart from introducing a new iPhone migration utility, iOS 12.4 adds support for Apple Card which is due to launch soon.
There really is no reason for any iPhone or iPad user to downgrade their device back to iOS 12.3.1/2. Even folks interested in jailbreaking their iOS device are out of luck since tools like Chimera and Unc0ver only support iOS 12 – iOS 12.2. The chances of an iOS 12.2- iOS 12.4 jailbreak being released are pretty slim.
iOS 12.4 is the likely the last release of iOS 12 as Apple shifts its focus to iOS 13 due to launch in September.