Git.io will completely stop working and redirecting URL from April 29, 2022. This came as the final announcement from Github today. In an announcement post, GitHub has asked any existing user to stop using the links provided by Git.io, as it will not work anymore.
Git.io was a URL shortening service provided by GitHub. The service was for shortening the URL of Github services. You can check the usage here https://github.blog/2011-11-10-git-io-github-url-shortener/. Starting from 2011, it was serving to date.
A few days back on Jan 11, 2022, GitHub announced that no new URL will be accepted in the service.
The reason was, that the usage of this service was not what GitHub expected it to be, as it was being used for malicious purposes.
So if you have used the service and provided the generate short URL somewhere, make sure to replace those link(s).