The updated rules can be found in Apple's App Store Review Guidelines under the "Hardware Compatibility" heading. Apple clearly states that power efficiency should be a clear goal for developers, given the battery constraints of a portable device like an iPad or iPhone (but especially the iPhone). "Apps should not rapidly drain battery, generate excessive heat, or put unnecessary strain on device resources," writes Apple. "Apps, including any third party advertisements displayed within them, may not run unrelated background processes, such as cryptocurrency mining."
That is the first shot at cryptocurrency apps; but Apple goes even further as you delve deeper into the App Store Review Guidelines. There is a whole section -- 3.1.5 (b) -- devoted entirely to cryptocurrencies. Apple has no problem with cryptocurrency wallets, exchanges, or initial coin offerings as long as the apps are provided by established and reputable institutions that comply with all "applicable laws".
However, Apple specifically calls out these two instances that it will not accept under any conditions:
(ii) Mining: Apps may not mine for cryptocurrencies unless the processing is performed off device (e.g. cloud-based mining).
(v) Cryptocurrency apps may not offer currency for completing tasks, such as downloading other apps, encouraging other users to download, posting to social networks, etc.
It shouldn't come as a surprise that Apple would crack down on apps that have the ability to seriously chew through your iPad or iPhone battery. We also have to keep in mind that Apple has just recently weathered Throttlegate, in which the company was found to have masked the natural degradation of lithium-ion batteries in its iPhones by throttling processor performance (without informing customers).