Once a user has installed macOS Sequoia 15.3, iOS 18.3, or iPadOS 18.3 on their device, Apple Intelligence will be automatically enabled on compatible devices.
From Apple’s developer release notes:
For users new or upgrading to iOS 18.3, Apple Intelligence will be enabled automatically during iPhone onboarding. Users will have access to Apple Intelligence features after setting up their devices. To disable Apple Intelligence, users will need to navigate to the Apple Intelligence & Siri Settings pane and turn off the Apple Intelligence toggle. This will disable Apple Intelligence features on their device.
Previously, users had to opt-in to enable Apple Intelligence by manually turning it on in the Settings app. From now on, the feature will be enabled by default, and users who do not want to use the feature will need to elect to turn it off.
Apple provided developers and public beta testers with the release candidate versions of iOS 18.3, iPadOS 18.3, and macOS Sequoia 15.3 on Tuesday, which means we’ll likely see the software updates be released to the public as soon as next week.