One of my favorite games ever, and I'd gladly pay for an ad-free version, but they don't have it. I assume there's much more money to be made with ads, over subscriptions.
Gameboy emulators are now available again in the iOS app store or even as a a web app. Or just play on real hardware.
I think a few bucks for ad-free brings in more money for most developers, but that still pales in comparison to a recurring subscription model (or models where people keep spending in-app on consumables).
Once governments force Apple to allow sideloading we should get some kind of f-droid for ios which only lists open source user respecting apps.