iPhone, iPad, and iPod touch owners who have upgraded to iOS 12.4.1 can no longer downgrade to iOS 12.4.
Apple routinely signs the signing of older versions of software updates after the release of new releases to encourage customers to keep their operating systems up-to-date. However, IOS 12.4.1 was released because iOS 12.4 had a major vulnerability.
The vulnerability allowed hackers to jailbreak iOS 12.4 and make devices susceptible to hacking attempts. Apple has fixed the bug in iOS 1
iOS 12.4.1 is now the only version of iOS that can be installed on iPhones and iPads. However, developers and public beta testers can download iOS 13, an upcoming update that is currently in beta testing and will be released next week.