Please release old/inactive usernames, especially those made during Justin.tv days.
This has been said to be under review almost three years ago (https://twitch.uservoice.com/forums/310228-account-management-e-g-login-connections-pass/suggestions/7956093-free-inactive-usernames-after-a-time-period). There are a ton of usernames that were made during Justin.tv days that haven't been used since. Even just a one time purge of accounts that have been 3 or 4 or 5 years inactive or more would be better than nothing just like what Microsoft did recently with Xbox Live accounts.
If you were to put a system I think Tumblr's is pretty good where they will email you a month or two in advanced that your user name will expire due to inactivity if you don't log in and it will be changed from e.g. Username to Username123 in order to release it for active users to use.