"This change will not affect your ability to play Xbox 360 games or DLC you have already purchased. Xbox 360 game content previously purchased will still be available to play , not only the Xbox 360 console but also Xbox One and Xbox Series X|S devices via backward compatibility."
I felt nostalgic and was playing Beatles Rock Band on my 360. The dlc I bought couldnt be accessed in-game or through my purchase history (which had a lot of content unavailable in it). Most other stuff was there it seemed.