As of September 15, Xbox now requires users to sign in at least once every 5 years to keep their Gamertag associated to their Microsoft account.
In 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.
If a user fails to do so, he or she might lose access to the gamertag associated with the Microsoft account, and that gamertag may become available for use by other XBL users.
At the moment of writing, the obligatory sign in only applies to a user’s Microsoft or Skype account.
For the full updated Microsoft Services Agreement, we suggest you read the full new Agreement right here.