Close to two years ago, I put out updates on Itch and treated it like Patreon, which was communicated on the Itch page. Several games (many popular ones) do this on this website even to this day.
Ever since I launched on Steam (Oct 2023), I've been selling the game as a one-time purchase. If you've spent $15 on an Elysium version on Itch at any point (back before Steam it was $20), you'll have access to the files now. No additional purchases are necessary.
Back then I used to sell the Endurance version, which was $5. It was missing, at this point, close to 15 scenes. I'm not selling that version anymore.