Since the Unity3D IAP team does not have the skills/interest in supporting UWP in-app purchases in the future I was wondering if there are any unofficial solutions available already?
“Agreed, Microsoft changed the API. Also, the market share for UWP is now less that 2%. We are looking to support other stores on the Windows platform like Steam. Closing this thread, UWP is an old topic.” said @JeffDUnity3D [UWP] Unity IAP Initialize failed without any callback
You could always call the Windows IAP APIs directly from your scripts.
@Tautvydas-Zilys Are there seriously no plans to update unity IAP so it works for UWP? UWP is still one of the avenues for getting a game on XBox outside of the ID@xbox program. Just saying it seems like an unfortunate decision.
I don’t know, IAP plugin was handled by a completely different team (I work on the core engine myself). @JeffDUnity3D would know for sure, and if he says so it’s likely the truth.
Yes, that is currently the case. Unfortunately we don’t have the resources.
IAP team should hire @Tautvydas-Zilys for one week. or probably one weekend would be enough to fix and improve current IAP Plugin 
https://assetstore.unity.com/packages/tools/integration/windows-store-native-54715 is a good asset for UWP In-app purchases