IAP have been shareable via family sharing since iOS14, assuming the developer opts in.
From what Patrick said, it seems IAP family sharing is turned on... which is good.
The individual app IAPs work like V1. They don't even need the affinity account to register the app.
My concern is specifically about the Universal License IAP since it needs an affinity account to tie the license to since you only buy the IAP in a specific app (say Mac version of Photos)... the iOS version of Photos doesn't know you have the IAP for Mac Photos...
Theoretically if the universal IAP can be shared, each family member could create their own affinity account and register the family shared IAP for the universal license to their own affinity account, but I am not sure if thats the intent (or if Affinity will registers a shared IAP like that).
Alternatively I could just log into my affinity account on their copies of the app, but it would be a pain to have to do that after every update if the account doesn't stay logged in after update, so a shareable IAP is easier...