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.
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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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Last year, there was a final Android 11 release for Pixels with the 2021-11-01 patch level before the Android 12 release with the 2021-11-05 patch level. It would be nice if that was done every year along with making the final previous branch release get the full 05 patch level.
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Could even make a strong argument for continuing it for a couple months beyond that and giving users a choice to temporarily delay the major OS update.
Many users are going to want to avoid Android 13 until apps work out compat ignores they neglected to deal with in beta, etc.
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At the moment, if users delay the major OS update at all, they don't get security updates.
Apple started giving users a window where they can stay on previous iOS version and still get security updates for a while, although it's probably a bit lacking in practice right now.
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We did our own 2022-08-01 update early this month (grapheneos.org/releases#20220) before Google released that as part of the August 15th release of Android 13 so we were ahead for a bit but now we need to quickly port to 13 for 2022-08-05 due to there not being an official August 12.1.
