s9e.github.io is an external website that's used by Flarum's TextFormatter (
s9e/TextFormatter) for some features like social embeds and maybe some sorts of code highlighting.
To my knowledge, its main use is making it possible to embed some external social websites that don't allow embedding cross-origin without some complicated configuration. That external server makes that process automatic for users of the TextFormatter library.
Can Malwarebytes provide you any more detail/reason? I don't see any public database of blocked sites on their own website so I can't look it up. From my quick search it looks like Malwarebytes might block some websites based on IP rather than hostname. Since that website is hosted with GitHub Pages, it likely shares IP addresses with millions of other websites, so it's possible a single bad actor using GitHub Pages caused the entire list of GitHub Pages sites to be blacklisted by IP.
If there's a legitimate concern with
s9e.github.io and not just its IP, it would be great to find the detailed explanation and report it to the maintainer so it can get fixed upstream. TextFormatter is an actively maintained project and Flarum also has interest in keeping it secure. But based on the information at hand it looks more like an invalid Malwarebytes flag.