I've updated the explanation of the Auditor app and attestation service at attestation.app/about to mention there's support for GrapheneOS in addition to the stock OS. Other OSes including full verified boot and keeping the security model for it intact can also be supported.
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Pixels are fairly unique in that they fully support verified boot and attestation for alternative OSes, but there are likely a few other devices doing it too. StrongBox HSM support (developer.android.com/training/artic) and Android Verified Boot 2.0 (android.googlesource.com/platform/exter) are also useful.
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