Hello! First of all, thanks so much for considering MDSOA.
The Deluxe Edition contains all of the routes, epilogues, plus version 01 of the art book (it will be updated later this year), the OST, as well as an additional mini DLC about some backstory lore of the game and also serves as a small epilogue to the game. There are also text-only POV short stories. It should all be available on Steam as well, Itch.io is just a bit cheaper, for the player and for me. I hope this clears it up, but let me know if you have any other questions 💙
https://aeondreamstudios.itch.io/Mystic-Destinies-Deluxe-Edition details it a little further.