The Android version that introduced Project Treble was Android 8.0 Oreo.
Announced in 2017, Project Treble reorganized Android’s architecture to make operating-system updates easier for device manufacturers. Before Treble, updating a phone often required hardware vendors and manufacturers to modify and retest large portions of the Android framework. Treble introduced a stable vendor interface separating device-specific hardware code from the core Android framework.
That separation meant manufacturers could update the Android portion without repeatedly coordinating major changes with silicon vendors. The goal was faster, less expensive updates and better long-term support, although Treble did not guarantee that every phone would receive updates quickly; manufacturers and carriers still controlled much of the release process.
Android 8.0 Oreo also introduced picture-in-picture mode, notification dots, notification channels, background-execution limits, adaptive icons, and the Autofill Framework. Project Treble should not be confused with the later Project Mainline initiative, which delivered selected system components through Google Play system updates.