The Persona 5 Royal DLC Bundle id free for the PS4 on the PlayStation Store. This is on the US PSN, which means that it will only work with an English version of the game. It includes the Kasumi Costume Bundle, the Battle Bundle, and the Persona Bundle. The bundle will also have “6 individual sets of DLC.” [Thanks, Wario64!]
When Atlus ported Persona 5 Royal onto other platforms, such as the Nintendo Switch, it came with all of the DLC. Aside from new costumes that you could put on your characters (which would play different music in battle), you could use certain Persona that don’t exist in the base game. These include references to past games, such as Izanagi and Orpheus. This may be why those with the PS4 version of the game were not eligible for a free or discounted upgrade to a PS5 version.
It is unclear if the free DLC bundle will only be available for a limited time. However, since the PlayStation Store page for it does not include a time limit, you will likely be able to download the pack until Sony terminates the PS4 PSN store or Atlus removes it.
Persona 5 Royal is available on the PS4, PS5, Windows PC, Xbox One, Xbox Series X, and Nintendo Switch. The DLC bundle is free on the US PlayStation Store for the PS4.
