Perhaps they are selling the domain, or planning to use it for a new service that allows users to create arbitrary URLs. Both of those scenarios can help an attacker take over old git.io URLs
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More likely IMO the backend is some old hacky thing that nobody has been maintaining and someone found a vuln or some software stopped getting security updates.
It's basically unimaginable that they would sell the domain. Whatever amount of money they'd get for it wouldn't be worth it given the potential risks to their customers (since it was formerly an URL shortener), and owning git.io is pretty on-brand for GitHub.