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Microsoft’s Secure Boot has been broken for a decade and no one noticed until now
(www.welivesecurity.com)
This is a most excellent place for technology news and articles.
That's moreso because it's using an unofficial key, so the device manufacturer (Google in the case of Pixels) cannot verify the authenticity of the OS you're running.
If you were able to replace that bootloader with a custom one, then you would be able to disable that message or just use a completely different bootloader like UBoot or EDK2 if it was ported, though.