Aged registered usernames

What happens to aged registered usernames? I installed Jami to see if the issues I had have been resolved. Needing to generate yet another username it got me thinking, how long do registered usernames last? I tried mine from about a year ago but was unavailable. Surely they must expire else there is a risk of running out of meaningful usernames?

Unable to test as the Windows 10 client crashes on startup, I suspect it is because of the VPN