Harmonix has issued a statement via the Rock Band Facebook page informing people that some tracks from the extended, downloadable Rock Band library are being removed from stores across platforms. This is due to licensing agreements reaching their expiration dates. Harmonix has stated that their team is working to get extensions on many of these licensing deals, but some tracks have already been removed. So far the only DLC tracks to disappear are Metallica’s “Ride The Lightning”, “And Justice For All” and “Blackened”. Harmonix has assured users that if you’ve purchased those songs, nothing will change for you. Even if you need to re-download them at a later stage you should still be able to, they just won’t be available for new purchases.
Source: Facebook (Rock Band Page)