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Anti-Rollback protects security-sensitive bootloader components from being replaced with firmware carrying an older rollback index. The warning in the OTA tools is based on the selected OTA package itself.
The archive extracts xbl_config.img from the selected OTA and reads its Anti-Rollback index. An ARB 1 warning means that this exact OTA contains rollback index 1.
No ARB 1 warning is not a guarantee of ARB 0. Future or manufacturer-specific implementations may not yet be detectable.
After a device enforces rollback index 1, downgrading it with bootloader firmware carrying the older index 0 will hard-brick the device.
OnePlus 15 · Chinese hardware
If you have a Chinese OnePlus 15 that was converted to OxygenOS and the bootloader was locked again, I strongly recommend restoring bootloader/Fastboot access now while the free method is still available.
You do not need to unlock the bootloader afterward. The important part is restoring access to the real bootloader/Fastboot mode.
If your device is affected, do this sooner rather than later.
Huge thanks to mikoker for discovering this method.