Respectfully, I don't think that's correct, or possibly I am misreading your comment. IIUC, placing a key in /usr/share/keyrings does not allow those keys to sign any package, only the packages designated with "signed-by" in the apt list.

Sadly, plenty of applications still take the old "apt-key" approach of adding the keys globally (e.g., installing keys to /etc/apt/trusted.gpg.d), but I think Signal's installation process is the correct/recommended approach for distributing apt packages securely.