iOS 11.3 is out, but only for the new 9.7" iPad
This is arguably the most important iOS 11 update yet, as it addresses a number of issues, such as the security flaw that allows Siri to read private notifications aloud, and introduces new battery management settings that provide users with more granular information about their batteries. The update also marks the debut of ARKit 1.5 and a number of other features, all of which you can check out here.
As to why Apple seeded iOS 11.3 for the new iPad only and nothing else, it's anyone's guess. The developer preview of 11.3 has been out since January, while the public beta has been available for weeks.
via MacRumors
Things that are NOT allowed: