Apps may not mine for cryptocurrencies unless the processing is performed off device (e.g. cloud-based mining).
In an update to Apple's developer policies, the tech giant has now stipulated that any app containing "background processes" which are unrelated to the app - such as cryptocurrency mining - are now specifically banned.
In March 2018, 9to5Macreported that Apple pulled a calendar app from its App Store for violating the efficient power guidelines.
Apple Publishes Revised Cryptocurrency, ICO Guidelines for App Store
The guidelines do allow apps that facilitate ICOs, as long as they work with established banks and follow all applicable compliance laws.
Apps related to cryptocurrencies, such as Bitcoin wallets and the accessing of exchanges will still be allowed on iPads and iPhones, so Apple isn't turning its back on the surging interest in cryptocurrencies.
The new Apple Books app launches this fall, with the Book Store available in 51 countries, and free books available in 155 countries. However, only official digital wallets providers and exchanges will be able to post their apps, potentially cutting out shady third-party software from facilitating cryptocurrency transactions.
Finally, the world's most valuable company has also mandated that apps do not offer cryptocurrency as rewards to users for completing tasks.