Previewed at WWDC, launching in the fall.
Apple Seeds Second Beta of iOS 10.3.3 to Developers [Update: Public Beta Available]
Registered developers can download iOS 10.3.3 beta 2 from the Apple Developer Center or over-the-air with the proper configuration profile is installed.
No significant features or notable bug fixes were found in the first beta, suggesting iOS 10.3.3, like iOS 10.3.2, is an update that's minor in scale and designed to introduce security improvements and address bugs. It does introduce new wallpapers for the 12.9-inch iPad Pro, though.
iOS 10.3.3 could be one of the last updates to iOS 10, as Apple is beginning to shift its focus to iOS 11. iOS 11 will make its debut at the upcoming Worldwide Developers Conference, which kicks off on June 5. iOS 11 will be in testing for several months before seeing a fall public release.
Update: Apple has also seeded a second iOS 10.3.3 beta for public beta testers.