Apple just had its huge event, and the company announced a new iPad Air, a new iPhone SE, and new colors for iPhone 13. The company tucked the release window for iOS 15.4 and iPad OS 15.4 in the press release for those devices, and we won't have to wait long.
In Apple's press release for the iPhone 13 colors, Apple addressed iOS 15.4 and when it would launch. At the very bottom of the press release, Apple said, "iOS 15.4 will be available as a free software update starting next week."
The phones, which come with the update, are slated to ship on Friday, March 18. 2022, so it might make sense for Apple to release the new version of iOS on the same day.
As far as iPad, Apple said, "With next week's release of iPadOS 15.4 and macOS 12.3, Universal Control will enable users to work with a single mouse and keyboard and move between Mac and iPad for a seamless experience, with no setup required." Again, the company specifically mentioned "next week's release," so we can be confident that we'll be able to take advantage of the cool features offered soon.
The company also mentioned that macOS 12.3 would launch next week. If you're a Mac user, you can expect to get some new goodies for your device, too.
Unfortunately, Apple didn't reveal any information about the release of watchOS 8.5 or tvOS 15.4, but the company tends to release significant software updates for all of its devices at once, so those may launch next week, as well.