Not that anyone would seriously use an iPhone to mine cryptocurrencies. Anyhow, Apple has updated its App Store Review Guidelines to explicitly ban the mining of cryptocurrencies on its iOS devices. An updated clause 2.4.2 now reads:
Design your app to use power efficiently. Apps should not rapidly drain battery, generate excessive heat, or put unnecessary strain on device resources. Apps, including any third-party advertisements displayed within them, may not run unrelated background processes, such as cryptocurrency mining.
It’s highly unlikely even the latest iPhone would have enough power to mine a single bitcoin, so Apple’s move seems more like a preemptive move to notify developers that such apps, which likely wouldn’t work and would only drain an iPhone’s resources, are a nonstarter. In other words: Don’t waste your time building them.