I disagree. If LineageOS builds were actually unsigned, I would have no way of verifying that release N was signed by the same private-key-bearing entity that signed release N-1, which I happen to have installed. It could be construed as the effective difference between a Trust On First Use (TOFU) vs. a Certificate Authority (CA) style ecosystem. I hope you can agree that TOFU is worth MUCH more than having no assurance about (continued) authorship at all.