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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 reads the Anti-Rollback index from the package's xbl_config.img. The number in an ARB warning is the detected rollback index of that exact firmware: for example, ARB 1 or ARB 3.
No ARB warning is not a guarantee of ARB 0. Future or manufacturer-specific implementations may not yet be detectable.
Once a device enforces a rollback index, downgrading it with bootloader firmware carrying any lower index will hard-brick the device. With ARB 3 enforced, both ARB 1 and ARB 0 are lower indexes.
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.
(Patched in 16.0.10.601)