Could we just publish (and cache!) TXT records at the appropriate-level domains? E.g.:
host -t TXT github.io.
etld_depth=2badguy.com would set the same flag, allowing a.badguy.com, b.badguy.com, c.badguy.com to count as fully independent sites
(Vouched so I could reply.)
Why would that be a bad thing? What happens under badguy.com is badguy.com's business, is it not?
Is it any different than setting cookie scoping rules?
The cookie use case is just one use case. Another PSL use case is helping browsers decide what part of the FQDN is most important for the user to notice. On firefox I see "ycombinator.com" in bold, with normal font weight on the news subdomain. On smaller screens the URL bar isn't left aligned, it adjusts the view to prioritize showing the relevant domain name. You don't want an attacker to control how the URL is displayed in that security context.
It’s also strongly used by chrome and similar browsers so malware at randomuserdomain.geocities.com doesn’t block the entire root domain.