Guitar Hero announced a discontinuation of DLC on their Facebook page, saying:
Sad to say that the downloadable songs for the Guitar Hero, DJ Hero and Band Hero games, and the Guitar Hero mobile game will no longer be available after March 31, 2014. The Guitar Hero servers will stay online and any previously purchased downloadable content is unaffected by this change.
We want to thank all of you for supporting Guitar Hero over the last nine years. To celebrate this great community, selected songs and tracks will be available for as much as a 50% discount on Xbox 360, PlayStation 3, and Wii from now until the end of the month.
For more information, please visit the store on your console.
Also, over at the iTunes store, the mobile game will also be available at a special discount until the end of the month.