If you hopped on to the Rock Band Music Store today, planning to buy "Ride the Lightning," you may have already missed your chance. That song, and the other three tracks making up "Metallica Track Pack 01" have been removed from the Rock Band Music Store following the end of term limits in the licensing agreements which allow for sale of the content.
In a statement published on the Rock Band Facebook page, Harmonix addressed the issue, saying that they are working to extend as many of the licensing agreements made for distribution of Rock Band DLC as possible, but that some other releases from the early days of the franchise may be unavailable for sale in future.
The removal and/or relicensing of songs from the marketplace should have no effect on users who have already purchased the DLC content.
Here's a half hour of the Arkham Origins Mr. Freeze DLC, if you want to watch that
11:45 AM on 04.15.2014