How can users who have purchased apps on Itch still get updates?
I can't answer that, sorry.
I'd guess, wait for the 1.1.5 update, then take a look here and on Patreon if the new version should be available on itch and if it actually is. If it turns out that Siren can't update the game here on itch while it's delisted, then the only other way would be to purchase the game again on Patreon.