Apple has updated its App Store guidelines to disallow all apps from supporting on-device cryptocurrency mining. Henceforth, any app that includes cryptocurrency mining feature will be removed from the store and no new submissions will be accepted.
The new rule, which was added some time recently, states "Apps may not mine for cryptocurrencies unless the processing is performed off device." Also, "Apps, including any third party advertisements displayed within them, may not run unrelated background processes, such as cryptocurrency mining."
The new guidelines may have been imposed over concerns regarding the longevity of the hardware and the battery. Cryptocurrency mining is a processor intensive activity that puts great demand on the hardware and the power supply and could affect the life of both. Users casually running these applications after hearing about the cryptocurrency craze may not be fully aware of what they are getting into. Apple may have already had to face devices damaged by mining, which is probably why the company decided to act so strictly on the matter.
It's worth mentioning that crypto mining on smartphones is a rather fruitless activity to begin with as the processing power of these devices is not enough to finish the task quickly enough to earn enough out of it. Your device will be under load constantly for little to no reward so it's not really worth it in the long run. Even desktop mining is waning as people are realizing the ASICs are the only way to mine efficiently so smartphone mining is nothing more than snake oil at this point. So it's probably for the best that Apple has put a restriction on it so fewer people fall for what is essentially a scam and damage their devices.