iOS 18.3 doesn't add any significant new features to Apple Intelligence—instead, it tweaks what's already there. Whereas Apple Intelligence was opt-in in previous OS versions, it is now on by default in iOS 18.3 on supported devices.

When installing macOS Sequoia 15.3, iOS 18.3, or iPadOS 18.3, Apple Intelligence will be turned on automatically on compatible devices, Apple says

Apple has kicked off iOS 18.3, iPadOS 18.3, and MacOS 15.3 with one significant change affecting AI. Since its debut, Apple Intelligence has required the extra step of actually turning it on or opting in. But with the new versions, Apple's AI feature is enabled by default for eliglble devices.