I agree that this is an issue, but it is impossible to prove a negative. The same could be said for Apple's or other manufacturer's signing keys. Who guarantees that the US government hasn't required access to the iOS signing keys? Or China in exchange for access to the Chinese market? They probably wouldn't even want to reveal that the signing keys were leaked if they were allowed to, since it would undermine their security story.

With a non-profit project of highly principled security experts, there is at least a high probability that they'd rather blow up the project than compromise. People elsewhere in the thread criticize Micay because he deleted the CopperheadOS keys, but to me it increases trust in the GrapheneOS project, since he clearly puts the security of his users over money, fear, and whatnot.

In the end trust arises from running a project or company long-term without evidence that you somehow compromised security.

I wonder in general how this situation could be improved. Second or third independent reproducible build + confirmation signing?