Unsure I really understand the XSS risk that's discussed in the links you provided. Nobody cares about that origin because it has nothing of value hosted on it and it's not like you'd be able to access cookies from a different origin.
The worst you could do is exploit a browser zero-day, but you can do that on any static hosting site already!
First, it's not hard to imagine that someone might try to get their account banned for a GitHub terms of service violation keeping in mind that GitHub holds the account owner accountable for content in their repository. This is true even if that content is from other account holders they've given access to their repository. In this case, anonymous access is intentionally being provided which could of course go very, very, very wrong.
"You agree that you will not under any circumstances upload, post, host, or transmit any content that:
is unlawful or promotes unlawful activities;
is or contains sexually obscene content;
is libelous, defamatory, or fraudulent;
is discriminatory or abusive toward any individual or group;
gratuitously depicts or glorifies violence, including violent images;
contains or installs any active malware or exploits, or uses our platform for exploit delivery (such as part of a command and control system); or
infringes on any proprietary right of any party, including patent, trademark, trade secret, copyright, right of publicity, or other rights."
Understanding what the tool does, GitHub might be forgiving on the ToS violation front. The problem is with the second scenario: law enforcement. It's very likely that in a lot of jurisdictions, law enforcement, prosecutors, etc., wouldn't initially understand what's going on here and even if it can be explained to their satisfaction, I think very few of us would like to spend a night (or more) in jail while attempting to explain.
You are abusing trust - now it's going to be the jstrieb.github.io who is serving malware, and since his system serves whatever JS I provide by design it becomes a very effective XSS host.
It's not really jstrieb.github.io that's serving it, because since the content is in the url fragment, it is never sent to or from the server, it's handled entirely clientside.
>a very effective XSS host.
It can only do XSS against jstrieb.github.io which has nothing valuable. So it's not useful for anything. It can't be used in a <script> tag to obfuscate XSS attacks against other websites either, because the response isn't formatted as javascript. I guess it could be used in <iframes> on other websites in order to add obfuscation, but I think the use to attackers would be quite low.
You still need something to serve you the initial document.write js, unless you are going to convince people to open your links with locally saved "index.html". I called it "XSS" because you can execute arbitrary javascript, and I was trying to avoid bluntly calling it "malware".
Though I probably should have. Here is an example of a HackerNews login page served with jstrieb.github.com https://tinyurl.com/yypvh3by, you can login to news.ycombinator.com with it, but it easily could have been a phishing site.
My point is, this is a very good idea for offensive operations.
But someone could register the github account newsycombinator and then serve an identical phishing page at newsycombinator.github.io .
I guess you're right that it's useful for takedown resistance in phishing attacks. It's useless for small, sophisticated, targeted phishing attacks, but for large blunt untargeted phishing attacks it could be useful to have a site that would be difficult to take down and censor.
The worst you could do is exploit a browser zero-day, but you can do that on any static hosting site already!