Searching for "PWA Paywall" on Google for itch.io. Happy to see interest on this topic.
PWA is developed by Google with some advanced surprises. So much it was adopted by Microsoft, up to including Edge being converted to Chrome. Safari is implementing the same innovations as well on iOS (slowly). And now PWA technology appears near end of incubation. With the Web App Manifest in "Working Draft" phase. After "Service Workers" was released in October 2018, on Safari. The final component to watch for full realization is the Web App Manifest:
For example- here is a PWA being used by the JS13KPWA team challenge: https://mdn.github.io/pwa-examples/js13kpwa/
It is the most advanced template found developed in collaboration with Mozilla.