Here's why Google won't let you downgrade to Android 12 after updating to Android 13 on your Pixel 6, 6 Pro, or 6a: There's a vulnerability in the old bootloader version that could be exploited if downgrading were allowed.
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This was what I thought was the reason that Google enabled anti-rollback with this update, but there wasn't any proof until this message, which btw, came from the Android Flash Tool.
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Here's a great thread on how anti-rollback counters work and why they're used:
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Some Pixel users are upset by the fact that the Pixel 6 cannot revert from Android 13 back to Android 12. I thought it might be helpful to write a short thread on anti-rollback counters, what they are and why they should be used a lot more. 
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Note BTW, that the message says *existing* Android 12 releases. I'm told that new images have been published that work with the incremented rollback counter. I have no specific knowledge of where those or published or how to use them and I encourage caution.
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Existing OS images already work on top of updated SoC firmware images. There could probably just be a final Android 12.1 release using the Android 13 SoC firmware images. We've mostly finished porting GrapheneOS to Android 13 now other than device support so we don't care much.
The issue with it being done in the major release is that projects based directly on AOSP like GrapheneOS don't have early access to source code so we can't have our stuff already ported on launch. It takes us a couple weeks after launch to port with us doing extremely hard work.
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Ideally, there would have been a final Android 12.1 release for Pixels with the 2022-08-05 Pixel patch level and updated rollback counter.
This would also benefit stock OS because a regular monthly release gets rolled out faster than a major release and users won't avoid it.
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