Github has changed their policy in 2022.

Before that it was possible to contact support to reclaim any username provided that they had no meaningful public repos and they were inactive for a long time. It was at the staff's discretion, there wasn't an elaborate policy of what constitutes inactive, but I've successfully reclaimed a username inactive for 2 years myself.

The old policy was:

    GitHub account names are provided on a first-come, first-served basis, and are intended for immediate and active use. Account names may not be inactively held for future use. GitHub account name squatting is prohibited. Inactive accounts may be renamed or removed by GitHub staff at their discretion. Keep in mind that not all activity on GitHub is publicly visible. Staff will not remove or rename any active account.

    Attempts to sell, buy, or solicit other forms of payment in exchange for account names are prohibited and may result in permanent account suspension.