Not confirmed that it's by Github, phishy domain.
Why is it phishy? Github.io has been the domain they use for all GH pages for a long time with subdomains mapping to GH usernames. It’s standard practice to separate user generated content from the main domain so that it doesn’t poison SEO.
First of all, any subdomain system domain is already a bit phishy because you need to somehow parse whether github.io is officially part of github.com and not say something like git-hub.xyz by a phisher or whatever new TLD there. These things are used by sysadmin/project pairs that can't budget 1$/month for a domain name, so it's 100% a security/price tradeoff.
Second of all, the actual domain host is publishing as one of these untrusted users on their alternate subdomain, so it could be a phisher using a subdomain of the official alternate domain with malicious material
Thirdly, even if it is all legit, it is still a problem, because it weakens security posture, it trains users to ignore domain names.
I understand if it appears subtle, but I wish that we lived in a world where whoever is responsible for this gets put on a PIP