I've successfully taken a username from a squatter by reaching out to github support. As long as the user doesn't have active private stuff they might be willing to help you get the username.
That's not an option any more. GitHub used to offer this, but as of about 12 months ago, the policy changed and they will not release a name that is inactive regardless of how long it has been inactive.
Using recent activity as a metric seems like it would age finished projects out of the github platform. Active projects that have switched version control several times in their history might refer to previous versions on github without github itself being aware of the connection.
At the free level I could see unmodified forks and anything marked private disappearing over time, then culling empty accounts. If I created an account, forked a project, and then left, how many months would need to pass before I'd be surprised that my account hadn't been recycled without my active consent?
I don't like calling them squatters if you don't know what they're doing and I'm pretty sure you can't actually tell if they're 100% inactive. For example at a time I had zero public activity on bitbucket but was using it for private repos and unlike github this wouldn't show. Then at some point someone kinda demanded I give up the name because I was obviously not using it. Same with Twitter. Just being logged in and reading has huge benefits since they broke the logged-out user experience.
Also archival purposes exist, especially for code repos.
