As the part of the company’s updated service agreement, Microsoft is going to clean house for Xbox Live Gamertag that haven't logged in from five years. This new agreement will go into effect from September 15 adding this new provision.
n the Microsoft Account or Skype Account section, we’ve added a requirement that you must sign into the Xbox Services at least once in a 5-year period to keep the Xbox Gamertag associated with your Microsoft account. We’ve also added that if your account is compromised, we may be required to disable access to certain content. - According to Microsoft
After losing your Gamertag Account, you’ll get a replacement of it as and when you make your login.
In earlier days also, Microsoft has done several exterminations to the disused Gamertag from Xbox. This step is also for automating that previous process of purging. But it might be very inconvenient for the masses who don’t use it on a regular basis.
If an account is inactive for so long, then it also makes sense for the username to go back into circulation.