I’ve personally talked with the creator. Have you? He does care. He needs to make a living, and getting paid for his work is important. You got what you paid for on itch. You didn’t pay for the Steam items. The creator didn’t know there would be Steam items, he wasn’t withholding them from you. Losing support and not getting free DLC are two very different things.
The creator’s not doing anything wrong here, and you need to stop thinking paying a small one time price gives you access to anything you want. You got what you paid for and you’re not losing it.