Strikes me as very dangerous though on the web where there are so many paths for malware to get in and this could get in the way of plugging the holes.
Secure boot can protect you eg. against malware gaining write access and modifying your system. I see it as user protection, as long as you can sign the trust chain. This is what GrapheneOS is doing as far as I know.
To take your GrapheneOS example, apps wishing to support it must add GrapheneOS keys: https://grapheneos.org/articles/attestation-compatibility-gu...
If this proposal goes ahead, it's unlikely that you'll be able to convince site owners and/or ad networks to add the keys of your open source OS.