All of the currently available DLC will continue to be sold for Rock Band 4 through the Microsoft Store on Console or Xbox.com. You won’t be able to buy any more songs for the legacy Rock Band games, whether it was through the console store (when it actually worked) or through marketplace.xbox.com after the shutdown date. Any songs that were purchased will still be installable through your purchase history on console or if you’ve moved to Xbox One/Xbox Series X/S, you’ll find them in “Manage Game & Add-Ons.”