Apple recently signed iOS 15.4 again for Studio Displays following update issues with the company's high-end display; Studio Display is unable to get updated to iOS 15.4.1.
The updates are available on all eligible devices over-the-air in the Settings app. It can be downloaded for free. Users may head to the Settings > General > Software Update to access the new software update. As per the update’s release notes, it fixes the issue that led to faster battery drains after updating to iOS 15.4. The update also brings a fix for Made for iPhone hearing devices and Braille devices.
iPhone 6s and later, iPad Pro (all models), iPad Air 2 and iPad 5th generation and later, iPad mini 4 and later, and iPod touch are all compatible with iOS 15.4.1 and iPadOS 15.4.1.