While Activision stopped development of Hero games quite some time ago, gamers were still able to buy DLC for their games. However, new DLC purchases won't be available come March 31st. The discontinuation is likely due to Activision no longer wanting to renew licenses associated with the DLC. For example, Rock Band developer Harmonix updates its fan base every quarter on the status of any potentially expired licenses.
While new purchases won't be supported after March 31st, any previously purchased DLC will be unaffected by the 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," a Facebook post (via Gamespot points out.