Since backward compatibility was updated, this DLC is still accessible on modern Xbox consoles (One, Series X|S) and the Xbox 360 marketplace.
If you own Prince of Persia (2008) on Steam, you likely have the Epilogue. Prince Of Persia Epilogue Download
Looking for the DLC? Before you search too hard, here is the current status and how to get it. Since backward compatibility was updated, this DLC is