This point-release probably gets iOS 12 ready for the Apple Card launch this summer.

Apple released iOS 12.3 in the second week of May, bringing us its new TV app with Channels subscriptions. Just days later, it started releasing betas to developers for another point release, iOS 12.4.

With WWDC just around the corner, and the first iOS 13 beta likely to land at that time, it’s not yet clear why Apple needs another release. Simple bug and security fixes generally don’t get major point-release status; we would get a 12.3.1 release if there were no significant user-facing features.
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https://www.macworld.com/article/3395824/apple-ios-12-4.html