Microsoft has recently updated their service agreement and a handful of changes were made to the Xbox Live section of the agreement. The biggest modification to the agreement is that there is now a set expiration period for gamertags. In order for a user to keep control of their gamertag, “[They] must sign into Xbox Services at least once in a five-year period, otherwise [they] may lose access to the gamertag associated with [their] account and that gamertag may become available for use by others.”
This marks the first time that Microsoft has made it explicitly clear to users the time frame in which a gamertag becomes available for other users to make their own.
Source: Service Agreement via Gamespot
