As per Apple Insider, Apple's crypto-related guidelines were originally created in 2014 after the app store unlisted Coinbase and other cryptocurrency apps stating some unresolved issue. While this particular adjustment may not seem to be a developer-friendly move at first glance because of its restrictive nature, it will actually be a boon for devs who create apps that offer cryptocurrency transactions. It also insisted that apps involved in Initial Coin Offerings, cryptocurrency futures trading and the like had to come from established banks, securities firms, futures commissions merchants or other approved financial institutions.
"Apps, including any third-party advertisements displayed within them, may not run unrelated background processes, such as cryptocurrency mining", the updated policy reads.
Cryptocurrency mining has been disallowed in App Store apps, unless the processing is performed somewhere other than an Apple device, such as a remote server or cloud provider.
However, since Mac users can download third-party programs from outside the Mac App Store, it's likely that mining will still occur on that platform without Apple's blessing. Ad networks can be compromised to serve coin-mining code, which isn't anything app developers can control.
The only limitation that Apple are putting down is that no apps will be allowed that encourage payments of cryptocurrencies or the incentivisation of cryptocurrencies to people who download certain apps or advertise certain cryptocurrency-based apps. This makes sense as in last December, Apple accidentally listed a fake version of the widely-used MyEtherWallet app in the App Store.
Along with new apps, the guidelines will apply to existing apps as well.
Similarly, Apple is also allowing apps to facilitate crypto transactions "on an approved exchange, provided they are offered by the exchange itself". Cryptocurrency apps can't offer users virtual coin for tasks like downloading other apps, getting other users to download the app, or boosting social media activity.