Earlier this summer, Microsoft released one million inactive Gamertags for Xbox Live to make it easier for people to get the names they want. Now they have updated their service agreement as well, and from now on you have to log into Xbox Live every five years if you want to keep your Gamertag. A not unreasonable demand to keep unused Gamertags in circulation, which hopefully leads to less people being named Killahbeezzz_1978.
It is worth noting that this is retroactive, so if you haven't signed in since 2011, now might be a good time to do so if you wish to keep your old gamertag. This is the new rule, straight from Microsoft themselves:
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.
The digital content you have purchased will still be available via your registered email, it's just the Gamertag that will go.