GrapheneOS 2019.09.23.19 release: grapheneos.org/releases#2019..
This will be the initial Stable channel release targeting Android 10.
Now that the initial port is done, work can begin on restoring past privacy/security features and implementing new ones. See github.com/GrapheneOS/os_.
Conversation
Hardware targets need to meet the baseline security standards including having full security updates and supporting all the standard hardware-based security features like verified boot with alternate operating systems. There are some potential candidates for support but not many.
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Please see grapheneos.org/#device-support. It also needs to be noted that no maintainers have stepped forward for the current devices and that needs to happen for support to be continued, let alone expanded. It's a collaborative open source project, and the community needs to step up.