The container registry has a horrible name.
https://www.reddit.com/r/webdev/comments/lg9xnm/why_do_some_...
It does not seem to hinder e.g. Google using google.com, youtube.com, gmail.com, and several (many?) others to collect your data. Do you say security and privacy work differently here?
In the case of user data domains, intentionally in the design of the service or via a security hole, users may be able to execute code and read cookies (e.g. in JavaScript on a page hosted on githubusercontent.com) and that's undesirable.
But if the different domain name gives good protection / isolation, why does Google still use completely different domains for different services with content controlled by them. I cannot believe they are interested in protecting users from data collection.