The entire publicsuffix thing is such a mess. The fact that project even exists is ridiculous. Why can this not be a dns setting?
One of the complications that gets discussed is that there are way too many use cases with different requirements on what it means for domains to be related. The cookie problem is something browsers need to solve, so its perhaps reasonable that Mozilla (and volunteers) manage a list that focuses on that user case. But that's poorly suited for other use cases.
There's a lot of people thinking about this though. One somewhat related example is https://datatracker.ietf.org/doc/draft-manelidis-dnsop-domai..., which has a double-opt-in DNS TXT record mechanism. That draft and forthcoming discussions on the mailing list are a good place to understand the complexities here.
because, like everything, bad guys would abuse it, causing a list of "bad publicsuffix" to exist, which is basically the same thing.
If, for example, you could trick a client into thinking "com" was not a public suffix, then from malicious.com you could set cookies on paypal.com
DNSSEC is not widely deployed, and so we cannot rely on the authenticity of DNS.
Maybe if we had DNSSEC, then we could rely on it for publicsuffix. Maybe lots of others things could be more easily solved if we had that too.
DNSSEC automation is coming, e.g. https://www.rfc-editor.org/info/rfc10026/. It might never become mainstream, but don't let anyone tell you that we couldn't benefit from having DNS authenticity!