Again, tygb, this is about iOS safari, not macOS safari.
The cookies in question fingerprint a browser, which is a unique identification of sorts, and they track you wherever you go even in private browsing mode. When you get into a website such as identity.com, which is in the business of selling information about you to anyone who asks, that's a real problem.
Here's a explainer article from 2015 about the flaw Apple has not fixed.
http://www.businessinsider.com/super-cookies-hsts-security-private-2015-1
"Apple users are particularly vulnerable, as their devices do not have a function that lets users delete super cookies from their browsers."
HSTS Supercookies cannot be deleted except by completely resetting the phone. What I am asking Apple to do at minimum is to provide a way to delete all cookies that actually works for all cookies.