Apple won’t allow it on Nostr apps until they have an incentive to do so

#Nostr is the protocol. However you access Nostr is the client. The LN (zap) button depends on the client. If you’re using the latest version of Damus there currently is no zap button because Apple demanded that it be removed of the app would be barred from the App Store. Other clients like Snort (Web), Amethyst (Android) etc still, of may still, have the zap button.

Even without the zap button on Damus, you can still send bitcoin to someone from their profile directly.

#Plebchain