For information on the present and future of GrapheneOS device support, see the official documentation:
grapheneos.org/#device-support
attestation.app/about#device-s
github.com/GrapheneOS/har
I get asked a lot of questions that are answered here. Lots of misinformation is also being spread...
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There's a guide on how you can participate in expanding device support for Auditor, by installing it on a device running the stock OS + bootloader locked and pressing a menu entry:
attestation.app/tutorial#expan
Every device with a valid sample submission has ended up being supported.
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In addition to being necessary for adding devices to the internal database of verified boot keys and other metadata in Auditor, these samples act as a security-focused hardware survey. It will play a large part in determining which devices are worth considering for GrapheneOS.